I'm going to write this short story someday:

A guy builds a time machine to go back in time and kill hitler. When he finds the young man, a struggling artist, he can't bring himself to do it. So he buys a painting instead, and tells Adolf he has promise, but not to give up.
Then he heads back home, beating himself up, only to discover that Hitler became an influential artist who was a bit of an anti-semitic douchebag, but who didn't go into politics. WWII and Nazism was therefore averted.

Eh. It's probably already been done.

Alternately, a guy who works in a lab in which multiple time machines are being tested goes back in time to kill or stop hitler and realizes if he succeeds, somebody far worse will rise in his place. So he spends the rest of the story protecting hitler (and hating himself for it) from his colleagues, who keep trying to come back in time, since he apparently failed. While the first few attempts are kept secret from the future fuhrer, one of the hero's colleagues almost succeeds. The hero's sacrificial efforts to stop him produces a change in hitler, who does not ultimately go into politics as a result.
Of course, that means the worse dictator will eventually rise to power, but hey, not every ending can be perfect.

...it's probably also been done. Anyway, there's my brainstorm for the day.