I wouldn't take the chance of accidentally fucking up the history even more. Maybe WW2 and the Holocaust were steps in a process that helped mankind avert an even greater catastrophe. Or I could create a paradox in which by killing Hitler I cause myself to not exist, so thus I can't kill Hitler which means I do exist which means I kill him, thus canceling my existence and making it so I can't kill him, which means he lives and I live, at which point I can kill him...
And even if I could avoid the above-mentioned cluster-fucks of paradoxes, I would rather just put him on a different path rather than kill him. Encourage him to get into that art school he wanted to go to perhaps? Maybe he'd just become a decent if somewhat anti-semitic artist. Plus there is a chance that some other demagogue would have risen to power instead who might have been even more horrible (or worse, more competent).
I believe that our timeline shouldn't be messed with, things CAN get worse than they are...