Objection 1. It would seem lawful for a man to kill himself. For murder is a sin in so far as it is contrary to justice. But no man can do an injustice to himself, as is proved in Ethic. v, 11. Therefore no man sins by killing himself.
Objection 2. Further, it is lawful, for one who exercises public authority, to kill evil-doers. Now he who exercises public authority is sometimes an evil-doer. Therefore he may lawfully kill himself.
Objection 3. Further, it is lawful for a man to suffer spontaneously a lesser danger that he may avoid a greater: thus it is lawful for a man to cut off a decayed limb even from himself, that he may save his whole body. Now sometimes a man, by killing himself, avoids a greater evil, for example an unhappy life, or the shame of sin. Therefore a man may kill himself.
Objection 4. Further, Samson killed himself, as related in Judges 16, and yet he is numbered among the saints (Hebrews 11). Therefore it is lawful for a man to kill himself.
Objection 5. Further, it is related (2 Maccabees 14:42) that a certain Razias killed himself, "choosing to die nobly rather than to fall into the hands of the wicked, and to suffer abuses unbecoming his noble birth." Now nothing that is done nobly and bravely is unlawful. Therefore suicide is not unlawful.
On the contrary, Augustine says (De Civ. Dei i, 20): "Hence it follows that the words 'Thou shalt not kill' refer to the killing of a man--not another man; therefore, not even thyself. For he who kills himself, kills nothing else than a man."
I answer that, It is altogether unlawful to kill oneself, for three reasons. First, because everything naturally loves itself, the result being that everything naturally keeps itself in being, and resists corruptions so far as it can. Wherefore suicide is contrary to the inclination of nature, and to charity whereby every man should love himself. Hence suicide is always a mortal sin, as being contrary to the natural law and to charity. Secondly, because every part, as such, belongs to the whole. Now every man is part of the community, and so, as such, he belongs to the community. Hence by killing himself he injures the community, as the Philosopher declares (Ethic. v, 11). Thirdly, because life is God's gift to man, and is subject to His power, Who kills and makes to live. Hence whoever takes his own life, sins against God, even as he who kills another's slave, sins against that slave's master, and as he who usurps to himself judgment of a matter not entrusted to him. For it belongs to God alone to pronounce sentence of death and life, according to Deuteronomy 32:39, "I will kill and I will make to live."
Reply to Objection 1. Murder is a sin, not only because it is contrary to justice, but also because it is opposed to charity which a man should have towards himself: in this respect suicide is a sin in relation to oneself. On relation to the community and to God, it is sinful, by reason also of its opposition to justice.
Reply to Objection 2. One who exercises public authority may lawfully put to death an evil-doer, since he can pass judgment on him. But no man is judge of himself. Wherefore it is not lawful for one who exercises public authority to put himself to death for any sin whatever: although he may lawfully commit himself to the judgment of others.
Reply to Objection 3. Man is made master of himself through his free-will: wherefore he can lawfully dispose of himself as to those matters which pertain to this life which is ruled by man's free-will. But the passage from this life to another and happier one is subject not to man's free-will but to the power of God. Hence it is not lawful for man to take his own life that he may pass to a happier life, nor that he may escape any unhappiness whatsoever of the present life, because the ultimate and most fearsome evil of this life is death, as the Philosopher states (Ethic. iii, 6). Therefore to bring death upon oneself in order to escape the other afflictions of this life, is to adopt a greater evil in order to avoid a lesser. On like manner it is unlawful to take one's own life on account of one's having committed a sin, both because by so doing one does oneself a very great injury, by depriving oneself of the time needful for repentance, and because it is not lawful to slay an evildoer except by the sentence of the public authority. Again it is unlawful for a woman to kill herself lest she be violated, because she ought not to commit on herself the very great sin of suicide, to avoid the lesser sir; of another. For she commits no sin in being violated by force, provided she does not consent, since "without consent of the mind there is no stain on the body," as the Blessed Lucy declared. Now it is evident that fornication and adultery are less grievous sins than taking a man's, especially one's own, life: since the latter is most grievous, because one injures oneself, to whom one owes the greatest love. Moreover it is most dangerous since no time is left wherein to expiate it by repentance. Again it is not lawful for anyone to take his own life for fear he should consent to sin, because "evil must not be done that good may come" (Romans 3:8) or that evil may be avoided especially if the evil be of small account and an uncertain event, for it is uncertain whether one will at some future time consent to a sin, since God is able to deliver man from sin under any temptation whatever.
Reply to Objection 4. As Augustine says (De Civ. Dei i, 21), "not even Samson is to be excused that he crushed himself together with his enemies under the ruins of the house, except the Holy Ghost, Who had wrought many wonders through him, had secretly commanded him to do this." He assigns the same reason in the case of certain holy women, who at the time of persecution took their own lives, and who are commemorated by the Church.
Reply to Objection 5. It belongs to fortitude that a man does not shrink from being slain by another, for the sake of the good of virtue, and that he may avoid sin. But that a man take his own life in order to avoid penal evils has indeed an appearance of fortitude (for which reason some, among whom was Razias, have killed themselves thinking to act from fortitude), yet it is not true fortitude, but rather a weakness of soul unable to bear penal evils, as the Philosopher (Ethic. iii, 7) and Augustine (De Civ. Dei 22,23) declare.
Source: Summa Theologica (New Advent)