Before embarking on a philosophical study of suicide, it may be helpful to attempt at least a preliminary definition of it.  A definition that comes to mind is the following:

A. Suicide is any form of self-killing, where self-killing is understood as acting in  such a way as to bring about one's own death.
 Problem: consider the following three cases:

1.  Jack gambles away his fortune.  So, he shoots himself.
2.  Joe climbs the Alps without a guide and dies (accidental death).
3.  Jim unwittingly drinks cafeteria coffee and dies.

Presumably, we agree that (1)-(3) are cases of self killing, but only (1) is a case   of suicide.  Hence, the definition is too wide.

4) Jim knows he'll be shot unless he betrays the underground; he doesn't betray and   is executed.
Presumably, (4) is not a case of suicide because Jim doesn't intend to die and doesn't see his death as a means not to betray.

5) Joe takes poison and dies in order not to reveal secrets.
This is a case of suicide because Joe intends to die not to reveal secrets; that is,  he views his death as a means to achieve a goal (keeping the secrets), and known  means to intended ends are themselves intended.
NOTE: Some, e.g., Beauchamp, deny this is a case of suicide because Joe is coerced and consequently doesn't act of his own free will (his act is not his own). On this view, neither Socrates nor Seneca committed suicide.

 6) Jack throws himself on a grenade to save his buddies and the explosion kills him.
 Presumably, this is not a case of suicide because
• Jack may believe that it is not his death which saves his buddies, but his covering the grenade.   Perhaps, with luck he might have survived while still achieving his goal.
•  his death is foreseen but not intended.

C. Another definition of suicide: X commits suicide iff:

1. X acts (or refrains form acting) in such a way as to bring about his own death.
2. X intends by those actions to bring about his own death either because  he   wants his own death per se, or because he wants something which he  thinks can   be caused only by his death (not merely by the act which also causes his death as   a foreseen but unintended consequence).

NOTE:
The definition allows:
•  possibility of coerced suicide (e.g., Seneca).
•  possibility of non self-inflicted suicide (e.g., loaded revolver in play or suicide by cop.)

 

Before investigating suicide, a few points are also worth noticing:

• It's far from clear whether suicide is a cowardly act, as it is often claimed. For one thing, it take some courage to commit suicide because it requires overcoming a natural instinct for self-preservation. For another, in some situation involving extreme suffering continuing life would be (perhaps foolishly?) heroic. But morality does not require us to be heroes at all costs.
• It's not clear whether the commandment "Don't kill" prohibits suicide. The problem is that in many circumstances we are morally permitted to kill, as in war, in self-defense, in protecting life and limbs. If I am allowed to kill you if I know that otherwise you're going to inject me with some horrible disease, why should I not be allowed to kill myself if I have that horrible disease?
• The issue of whether suicide is morally permissible is different from that of whether suicide is reasonable. Note that there are many unreasonable and stupid things that can be morally permissible.
• It is simply false that people who commit suicide are mentally ill, although no doubt some (and perhaps many) suicides are the result of mental illness. In many cases, the decision to commit suicide is reached with proper information and good reasoning.
• The idea that one should keep on living at all costs because there's always hope that things will get better is very problematic. Sure, it is possible that tomorrow someone will discover the cure for my horrible disease and give it to me. But our reasonable decisions are made on the basis of what is probable, not merely of what is possible. It's certaily possible that tomorrow I'll win the lottery, but it would be unreasonable of me to start making debts today because what is possible need not be probable.

Aquinas on suicide.

It's wrong to commit suicide (unless commanded by God!)  because:

• Suicide is contrary to natural inclination.

Objection: What does "natural" mean? As the inclination to commit suicide is not uncommon, whay is it not natural?
Reply: "natural" as normative notion stemming from an Aristotelian (perfectionist) consideration of humans: one ought to strive for happiness (the actualization of human capacities), and suicide prevents that.
Duplication: but then the good is happiness, not life per se, and if one's life is wretched and with no hope for happiness, the reply fails.
• Suicide damages the community.

Objection: not in all cases.
• Our lives property of God.

  Problems:

  • In what sense am I God's property? Is my being owned compatible with human dignity?
  • If it's my body that's God's property, why should I not be able to leave it at will, like one may leave a house that does not belong to one?

For Kant, the intention to kill oneself constitutes suicide.  He provides two order of arguments against the permissibility of suicide, secular and religious.   The latter dependent on the former: God forbids suicide because it is wrong, not vice versa.

A) Secular arguments
 

• The person who commits suicide acts on the basis of the maxim "shorten one's own  life from self-love when future bad more than good".  But then, the principle of  self-love, whose end is to preserve one's life, becomes the spring for one's death.

NOTE: The argument, as it is, is unclear.  Presumably, Kant means that the principle of  self-love becomes self-defeating.
Hence, the abstention from suicide is a perfect duty.
Problem:
The principle of self-love can be taken to involve two different things:
1. a blind instinct of self-preservation (whose task may be the preservation of life at all costs)
2. a calculated desire for pleasure and aversion to pain.
Kant’s point applies, if at all, to the first one.  But the spring of suicide can be the second.
 
• The person who commits suicide destroys his life by his free will, which is itself destroyed in the process.  But to use free will so as to bring about its own destruction is “self-contradictory”

NOTE: The argument is unclear.  Presumably, the idea is that using a human capacity to destroy that capacity (or life to destroy itself) is self-defeating.
Problem: In what sense is this self-defeating?  Clearly not in the sense in which  “lie when it's advantageous to do so” is self-defeating when universalized  (“commit suicide whenever life is unbearable” is not-self-defeating when  universalized).
 
• Suicide annuls the precondition for all duties, namely, the existence of the agent.

Problem: this is true, but why should this be morally significant?  Duty exists only for a subject; hence it's absurd to mourn the lack of duty-fulfillment once the  subject doesn't exist.
Reply: a person has intrinsic value because of its free rational will.  Destroying it to achieve something less worthwhile is irrational.
Duplication: brainwashing?  Cato, perhaps? Mental degeneration? Perhaps Kant ought to qualify his denunciation of suicide.
• He who's prepared to commit suicide has no respect for his life; hence he has no respect for the life of others and is not deterred by the fear of punishment.

Problem: this depends on whyone is prepared to commit suicide.  There's no  evidence that Seneca had no respect for the lives of others or was vicious because  he did not fear punishment.
 
• We have a duty towards ourselves because the humanity in us inviolable.  Human  nature has an intrinsic value which is negated by suicide, which involves treating  oneself as if one were a thing.

NOTES:
• Kant infers that, therefore, the suicide becomes “an object of free will” for  everyone.
• For Kant the preservation of one's life not highest duty.  Our duty is to obey the  moral law which preserves human dignity.  It's better to be killed than  renounce one's human dignity.
 Problem:
K. distinguishes between one's life and one's humanity;  duty towards the  former is  conditional, towards latter absolute.  But suicide at times originates in respect for  one's humanity (personhood), as in the case of suffering or living in chains.  See, e.g., Seneca's letter 70 to Lucilius, or even Kant's remarks on Cato's suicide (the only case about which he seems in doubt)
Reply: One's humanity cannot be diminished or stripped away because morality is  totally autonomous (one can always be moral in Kant’s view).
Duplication: It is far from clear that this is so.  Consider cases of forced administrations of drugs, brain-washing, concentration camps, mental degeneration, etc.  Ultimately, Kant fails to appreciate the importance of moral luck.

• Since God's intention is to preserve life, suicide is against God's will.

Problem: How do we know God's will?  If God wants to preserve human life, whence   disease?  Moreover, does a benevolent God really want to preserve life at all costs, even if one is suffering horribly?
• Suicide is a rebellion against God and a desertion of one's post.

Problem: To say that suicide is an act of rebellion, we ought to know God's will in each specific case; this brings us back to the previous case.  Talk of abandoning  one's post is, at best, a bad military analogy.
• We are God's property.


Problems:

• This too, at best, is merely metaphorical, as it's in tension with the notion of human dignity. 
• The same objections against the similar point in Aquinas apply here.

Hume on suicide.

Superstition and our natural fear of death make our views on suicide muddled.  It's the task of philosophy to free us from confusion and show that suicide may be free from all imputations of guilt and  blame because it is not a transgression of our duties towards God, society, or ourselves.

1. Suicide is not a transgression of duty towards God.
God is the creator of a system of inanimate and animate creatures which act on the  basis of immutable laws.  Human life, like everything else, is subject to natural  laws, and diverting a few ounces of blood from their usual course is no more an  encroachment on providence, or a disturbance of the order of creation, than  diverting a river. It's arbitrary to allow the former and condemn the latter.
NOTE: H's point is that human life, like most everything else, is rightly subject to human prudence.
Objections:

• Human life of so great importance that only divine providence should deal with it.

Reply:
1. the antecedent is false.  Human life not so important since a hair, a fly, an insect can kill us, and in relation to whole universe, a man's life not more important  than an oyster's.
2. the antecedent is immaterial.  Even if human life were very important, if only providence should deal with it, then it would be criminal to cure oneself to prolong one's life.
    
 
• Our lives not our own; they belong to God, and consequently suicide is a transgression of duty towards God.

Reply: Then heroic self-sacrifice of life, be it intended or not, is criminal, a view Hume doesn't accept.
 
• By suicide, we repine at providence.

Reply: On the contrary, I thank providence for the good in life and the capacity to commit suicide when life becomes unbearable.
Duplication: I've been placed here by God as a part of a divine providential plan. Abandoning my post, as it were, is a breach of duty.
Reply: Hume in effect gives a two barreled answer:
1. There's no evidence that I have been especially placed here: I owe my life to a chain of causes like everybody else.
2. If one objects that all causes are guided by providence (there is a providential order in the world as a whole), then providence is pervasive.  But then suicide is within its range and a part of it. Moreover, it is blasphemous to think that any human act can disrupt a divine plan.

• In retiring from life, one at most stops doing good, which, if it is a harm, is a small one.  Moreover, if one is a great burden to society, suicide would be laudable

Problem:  The first point seems false if one leaves debts or unfulfilled obligations; the second might have undesirable consequences, such as pushing to suicide people who want to live but are a burden.
• Nobody is morally bound to do good to society at the expense of a great harm to oneself.
• Upon infirmity one may justifiably retire from society.  Hence, upon a great evil may retire permanently.

3. Suicide is no transgression of duty to myself.  Age, misfortune, sickness can make  life worse than death.
 

Suicide and Paternalism

It is one thing to believe that suicide is morally unjustifiable, it is another limit the freedom of action of one who wants to commit suicide.  We value rational agency, and one of the manifestations of rational agency is acting in accordance with one's own conception of the good (i.e., autonomously).  In other words, the fact that an agent can act autonomously is good.  It follows, then, that one might be morally required to uphold one's legal right to do what's morally wrong if the morally wrong act is a manifestation of autonomy and compatible with it.

Note: this rules out things like self-enslavement.

Paternalism is the limitation of a person's liberty of action or information for the sake of that person's welfare or needs.  Since paternalism restricts autonomy, it requires a justification (it's guilty until proven innocent, as it were).
It helps to distinguish two types of paternalism:
 

1. Weak paternalism, namely interference in case of:
• uninformed decision: Romeo kills himself because he thinks that Juliet is dead,  while she is merely drugged.  Notice that we know that had Romeo known the truth he would not have committed suicide.

NOTE: there seems to be no reason here not to interfere in order to tell Romeo how things are.
• mental incapacity

NOTE: whether one is mentally incompetent is often hard to determine.  However, the claim that anyone who wants to commit suicide is mentally incompetent is excessive.
2. Strong paternalism, namely, interference when the agent seems to have made a decision to commit suicide on the basis of reasonable use of the relevant  information, and when all of one's values are taken into account.

NOTE:
•  importance of relevant information, especially concerning the future.
•  importance of its reasonable use; often (but not always) deep depression leads to misjudgment.
•  importance of consideration of all of one's values, not just those which seem temporarily prominent because, say, of depression.
• importance of present vs. future values.