Horrible to say it, but the only answer is to do nothing. Quite apart from the tragedies of those who died, can you imagine how instantly the world would change today if the 50 million people who died in WW2 had lived? And the descendents they never had? The world would be changed so radically, who knows what would happen? Other tyrants would have emerged, and good people would just cease to exist as their parents' histories would have been altered. In fact, half the people alive today in the countries involved would probably cease to exist because our parents and grandparents' lives would have followed different courses.
Maybe it would be nice to go back to just before he shot himself and have the satisfaction of telling him how history would judge him - that wouldn't change history.
It's academic anyway - you cannot go back in time! (But that's another topic!)