You know, I have to wonder why everyone comes up with violent solutions for if they had a time machine. Actually, given even minimal knowledge of history, it would be easier to tug a string here or there and watch it all unravel without any violence or showing everyone that an intervention just happened.
E.g., take Hitler, yeah... Why bother shooting him or enlisting or whatever, when he can be taken out of the equation gently before it even started? E.g., go back to right after WW1 and pay for some serious art lessons for the dude, so he doesn't get rejected by the Vienna arts academy, or at least can be a celebrity artist even if they reject him anyway. Or help him go to America and find a job with Disney, since he was a big fan of that and did have some talent at drawing.
He'd probably be happy enough and not end up blowing his brains out in a bunker, so, hey, everyone is happy.
ETA: Of course, things would probably still go downhill in Europe in some way or another, but probably more in a communist way than nazi.