I'd rather not mess with Baby Hitler, because stuff to do with WWII would probably prevent my grandparents meeting, therefore preventing my birth which results in me not having the time machine to kill Hitler and so on and so forth. So basically killing Hitler would have probably broken time.
I would, however, go into the future to see if there's a cancer cure yet and give it to Bill Hicks and George Harrison.
Oh, and I'd probably go and scare the shit out of my 7-year old self for laughs.
Dead parents or a destroyed home planet?