Well wothehell, let's throw Hitler and Stalin and Pol Pot into the mix as well.
No doubt a thorough postmodern psychological analysis would show that Voldemort is a product of complex historical and social forces beyond his control and not really master of his own fate, leading to the conclusion that there's no such thing as a bad wizard, only a sick one. Similar conclusions would be found for the others as well, and in fact was for Hitler at least, that's what led to the initial appeasement of him by the likes of the spineless Neville Chamberlain and a generally craven British establishment. Ultimately however, it doesn't really matter why they're the way they are, what matters is that they *are* the way they are and are responsible for what they do, as Churchill clearly saw in the case of Hitler: the Nazi dictator was irredeemably wicked and therefore had to be taken down, no compromise with iniquity.
Not the answer you expected, I'll wager.