Trump is very erratic and mercurial. If he were President for four years there would be plenty of opportunities for him to have a falling-out with Putin. His judgments of individuals can change in a flash based on how he perceives their attitude toward him. A clash with Russia couldn’t be ruled out.
Aside from that, if a nuclear strike ordered by the elected leader of the US killed millions of innocent people in (for example) the Middle East for no valid reason, the stain of that guilt would cling to our country for as long and as indelibly as it has clung to Germany and Japan. With the great power of our country comes great and inescapable responsibility upon us as voting citizens.
This is, I think, the ultimate issue at stake in the election. We've had more than a year now to watch Trump on the national political stage and see what his personality is like. We know his belligerent and impulsive nature and his grotesque ignorance and prejudices concerning the world outside the United States. And we know, or should, that whoever is President has personal control over an arsenal of about 7,200 nuclear weapons, many of them hundreds of times more powerful than the Hiroshima bomb.
What if Trump lost his temper and acted on impulse during a future feud with Iran, for example? The Tehran metropolitan area contains about sixteen million people. A single act of rage could produce a death toll two and a half times that of the Nazi holocaust. And the very name "American" would then be branded with that act for generations.
Or it might be somewhere else, not the Middle East at all. The man is nothing if not unpredictable.
Electing Trump would mean creating an intolerable risk of an act of mass murder to which we as voters would all be accessories. This is really not any more complicated than that.