The most philosophically plausible & the most Kantian argument Kant offers for his moral prohibition on suicide is in the “Methaphysics of Morals”. There he argues that because rational agency is the source of all moral worth, rational agents who kill themselves renounce the authority of morality for the sake of a deal more narrow than is generally believed, requiring only that we not frame for ourselves any maxim prescribing our own death as an end, though death may be a necessary means to some other morally obligatory end.