Suicide is self-murder.

On the authority of St. Augustine, in The City of God and in his epistles, suicide is error as impiety, an abusive reading of Holy Scripture. Suicide is sin according to the commandment and divine prohibition: Thou shall not kill. Suicide is a sin against oneself: to murder one’s own innocence and justice. Suicide is self-exile, expelling oneself “outside the church, separated from its bond of unity, its chain of love.” Suicide defies the calling of love.[1]

On the authority of Thomas Aquinas, in the Summa Theologica, suicide is a mortal sin against God. It is unnatural, for everything loves itself and endeavors only to preserve itself. It is unlawful, for everyone is a member of the community and suicide injures the very same. It is treachery against God, one’s maker and master.

Man is made master of his own free-will; and therefore may lawfully dispose of himself in things that relate to this life, which is ruled by the free will of man. But the passage from this life to another and happier one is not subject to free-will of man, but to the divine power; and therefore it is not lawful for a man to kill himself to pass to a happier life. Neither must he do so to escape any evils of the present life, because the extremest and most terrible of the evils of this life is death … and therefore to compass one’s own death in order to avoid the other miseries of this life, is to take the greater evil to escape the less.[2]

In sum: “no one is judge in his own cause” and no one may put himself to death. Such is the limit of one’s free will.[3] An echo and reaffirmation of Augustine, who writes: “one may not act on his own authority, in the matter of self-murder.”[4]

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[1] Augustine, City of God, 1.17 & 20; Augustine, Letters 173 & 204, in Political Writings.
[2] Aquinas, Summa Theologica, Second Part of the Second Part, Question 64, Article 5, Whether it is lawful to kill oneself.
[3] Ibid.
[4] City of God, 1.26.