The most obnoxious Democrat supporters have their whole ego caught up in being a “good person” by having the correct ideological positions. Admitting that their “team” is full of war criminals would directly challenge their egos and is thus summarily dismissed. They are idealists, and thus disconnected from material reality. They tend to be quite caught up in the notion of “electability” - the idea that they shouldn’t back candidates or platforms they actually want, but those calculated to appeal to the largest number, which is always smack dab in the center of the Overton window, in their estimation. (Of course, ironically, a true socialist platform would have the greatest appeal were not USians so propagandized against it.) I have literally had liberals tell me that though they are not themselves racist, they need a candidate that would be accepted by racists. I think more often than not, they cry “electability” so they have an excuse for supporting a conservative candidate while simultaneously calling themselves “progressive”.

This is to say nothing of the fact that liberalism is a conservative ideology, and that it is more than happy to back imperialism if it means the spoils can subsidize their lifestyle. Or in other words, Democrats will cheer for any war if it keeps their Starbucks cheap and their gas tank filled. They are not anti-war at all. They oppose the appearance of warmongering, but not the actions themselves; they want plausible deniability so they can go back to brunch.

Tl;dr - Democrats are bad people who pretend to be good people and back war criminals because they like imperialism, no matter what excuses they make for it.