Reminds me of a twilight zone episode. A woman goes back in time to kill Hitler as a baby. She successfully poses as a housekeeper or something and kidnaps the baby Adolf. She then jumps into some body of water, drowning herself and the baby. However, the nanny convinces a homeless woman to give her baby boy to her, and successfully passes that baby off as Adolf Hitler. However, that baby is mentally unstable, and grows up to be the Hitler we know. So, by killing the real Adolf Hitler, this time-traveler fulfills history.
If I could prevent one thing, it would probably be WWI. Not sure how I would go about stopping it, there were a number of indirect causes involved. Preventing WWI would prevent WWII, probably the Holocaust, the Cold War, etc.
There would have to be some negative consequences of altering any major event in history, which makes this a tricky question.