Suicide is unnatural.

Aquinas establishes that suicide is a sin against nature.[1] Suicide defies the law of self-preservation and propagation. Life strives to live, not to extinguish itself.

In Two Treatises of Government, John Locke concurs and elaborates. Man has liberty, though not the license to destroy himself.[2] “For man is called, by right and imperative, to the divine and fundamental command of self-preservation.”[3] Man thus violates his own right and imperative through suicide. Locke concludes: the “freedom of nature” is to be restrained by the “law of nature.”[4]

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[1] Aquinas, Summa II-II, Q64, A5.
[2] John Locke, Second Treatise §6.
[3] John Locke, First Treatise §86.
[4] Second Treatise §22-3.