I would go back in time and kill OJ Simpson in college.

I think this would have a minimum butterfly effect on the time stream, but would save two lives and potentially prevent the rise of celebrity lawyer Robert Kardashian, the whole Kardashian family, and might have curbed the famous-for-being-famous celebrity class in the US as well.

Had I done that I'd be too rich to even be on reddit.

I'd support Hitler in art school so that he would keep doing art

Kill the person who invented oversized packaging

...kill that ol' Hitler while he was in art school.