If you had a chance to go back in time and kill Adolph Hitler before he could kill six million, would you do it? That one is pretty simple. Most of us can imagine that scenario. I would prefer to go back in time and buy one of his paintings. "Hey, maybe I can give this art thing a try." The Fuhrer might mutter, "That world domination thing can wait." The world loses one of its historical evil masterminds and gains a starving artist.
No World War Two? Maybe then we have no Stalin either. Old Joe would be stuck back in Russia trying to keep the motherland together and becomes a bureaucrat mired in the business of the post-revolution. The Cold War is pushed back decades while the Soviet Union tries to find its bearings without a war to wage.
Without a Second World War, there wouldn't be a need for the Manhattan Project. Hiroshima and Nagasaki remain intact as Robert Oppenheimer turns his attention to a fusion reactor that can power cities. Harry Truman spends more time playing piano and taking walks. Dwight Eisenhower doesn't become president because he didn't have a D-Day. He doesn't pick Richard Nixon out of relative obscurity to be his vice president.
No Watergate. You're welcome.
Oh, and Richard Nixon doesn't end up making friends with Hafez al-Assad, president of Syria. Father of Bashar, who is president now. In Syria, where the world finds itself picking sides and taking aim at one another over the use of chemical weapons, another World War hangs in the balance.
All of this could have been avoided if someone would have given that little corporal a chance at making a career for himself painting canvas after canvas in his studio apartment. He won't have to blow his head off, but if he did, it would be because he was a tortured soul who was misunderstood by his patrons. Not because he was overcome by guilt and fear as the Allies crept closer to his bunker.
So, if you're working on a time machine, remember to set a little cash aside from that big grant you have been rewarded for doing all that theoretical research. Not to buy a gun, but a painting.