Listers, the following is the body of the question on “Whether it is lawful to kill oneself?” by St. Thomas Aquinas in his Summa Theologica. The Angelic Doctor speaks of suicide under the greater question of Murder, listed under those acts contrary to the virtue of Justice. Everything henceforward is quoted from the body of ST II-II.64.5 save the addition of the titles by SPL.
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.
1. Contrary to Natural Law & to Charity
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.
2. Injury to the Common Good
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).
3. Sin Against God
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.”