I dunno I mean it's a pretty different proposal to logically come to the idea of like. oh something that's an existential threat needs to be nuked, whether that be like, godzilla, which is NOT a good idea, or like, just ensuring MAD because fuck it, I guess, or like, north korea or something, right, there's a difference between that, which should probably be a pretty cold and calculated decision, and like, killing someone, presumably that you know quite well after travelling with them for you know, at most, eight years, and then rooting through their corpse to find a little code with what's probably a time critical objective. I think probably, as another commenter pointed out, you would want to elect the president that can't kill a person. That's a better president, than the one that can, probably.

Bigger hole than all that, though: the president would probably just ask the secret service to do it, and the secret service would probably comply.