Depends on how you handle it. If he's just a side character in another story, you could have the main character encounter him, listen to him talk about all his guilt and such, and then totally ignore all of the big disasters of history to bitch at the guy for causing his personal problems.
It has been done on a smaller scale. Arthur Dent, from Hitchhikers Guide, is responsible for the death of a person through every single one of their reincarnations, which ranges from a fly, to a rabbit, to a bowl of petunias
So, it's possible, you just need to make the world odd enough to allow it.
@Mr AHR: I mentioned that in my first post, actually.
Not nearly crazy enough. I'm talking about a grand, glorious cluster of insanity involving time warps, time loops, aliens, and who knows what else. Simply saving Hitler is far too mundane.
Save a webcomic, read Commander Kitty!
"You got your head all tangled up, but if I could only
make you care..."