Ask anybody what they'd do if they had a time machine, and the first thing they say is "Go back in time and kill Hitler".
Yeah, but what if Hitler had a time machine?
What if you go back in time to kill Hitler, Hitler gets the better of you with the old surprise wedgie and he runs off with your time machine? What's he going to do?
My guess: Go back in time, ensure Germany loses World War I, falls into economic ruin, motivating the people to elect an extreme Nationalist party that revives the country's economic and military might to become the most feared state in the world, capable of virtually anything it sets it's expansionist, murderous mind to achieve.
Say what you like about Hitler. At least he killed Hitler.
And while we're on the subject, why is it always Hitler? Why not world's worst person Josef Stalin? Why not world's second worst person Jimmy Savile? First thing I'm going to do is go back in time and kick my arch-nemesis the Dalai Lama (Not daily. Not a llama) in the fork. No reason, it would just be really funny. Then Mick Hucknall.