Before embarking on a philosophical study of suicide, it may be helpful to attempt at least a preliminary definition of it.  X commits suicide just in case:

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.
   Note that foreseeing is not intending.
   

A few more points are also worth noticing:

1. 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 heros at all costs.

2. It's not clear whether the commandment "Don't kill" prohibits suicide. Certainly, the commandment is not "Don't kill other people" and therefore it renders suicide guilty until proven innocent, as it were, Still, 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?

3. 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.

4. 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.

5. 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.

6. Although some, perhaps many, cases of suicide do show disregard for the feelings or the interests of family and friends, it's simply false that all do. Sometimes people commit suicide with the agreement of family and friends, and sometimes they take into due consideration not only their own feelings and interests but also the ones of others. Hence, suicide need not be a selfish act any more than most other acts.

Aquinas on suicide.

The argumentative pattern Aquinas follows is that of a medieval disputation. First, he raises the question of whether suicide is permissible. Then he presents his opponents' arguments for an affermative answer. Then, he states his own view (that suicide is not permissible) and provides his arguments for it. Finally, he answers the arguments of his opponents. Here we focus only on some of his own arguments.

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

• Suicide is contrary to natural inclination.

Objection: what about the natural inclination to commit suicide?
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 are the property of God.

For Kant, the successful intention to kill oneself constitutes suicide.  He provides two kind 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.  Hence, we concentrate on some of his secular arguments.

• The person who commits suicide acts on the basis of the maxim "shorten one's own  life from self-love when the future is very bleak".  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.
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 is usually the second.
 
 
• 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: Destroying an end in itself to achieve an end concerning passions (e.e., the termination of pain) is contrary to CI
 
 
• 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 the respect for one's human dignity which may be degraded by misfortune (e.g., unbearable suffering, living in chains, brainwashing, etc.)
Reply: One's human dignity 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, etc.  Ultimately, Kant fails to appreciate the importance of moral luck.

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.
Hume's general strategy here is to argue that human life, like all other things in the world, is rightfully subject to human prudence.

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.
NOTE: the point seems to be that it's arbitrary to allow the former and condemn the latter.
Objections:

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

Reply:
1. the antecedent is false.  Human life does not seem that important since God allows a hair, a fly, an insect to kill us; moreover, in relation to whole universe, a man's life is probably 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 are not our own; they belong to God, and consequently suicide is a transgression of duty towards God.

Reply: Then some cases of heroic and much admired self-sacrifice of life would become morally wrong..

• By committing suicide, I 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 one may retire permanently.

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.
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 rational 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.