Saving the world?

So being able to time travel doesn't sound too convincing to me. First of all, I'm not so keen on changing the aspects of the past. I think most of us have seen "Back to the Future", where the effects of changes were worse than we could've ever expected. Recently I've also read Stephen King's newest book "11/22/63" - talking bout travelling in time to avoid Kennedy's assasination, and honestly, the consequences of saving his life were quite... interesting (not to spoil for those, who decide to read the book). 

But if I absolutely had to change something I'd make sure Hitler never gets power over Germany, this way I could, hopefully, prevent WWII from breaking out. I'd use whatever methods I'd need to, maybe even hire an assassin... Sounds cruel, but saving 72 million lives for the cost of one? I think this decision is quite easy.