I've contemplated this random thought before.
While killing Hitler before the war would save millions of lives, it would also change or possible negate several million more.
The millions of immigrants who came to the US right before, during, and after World War II would have been less likely to leave a peaceful Europe. That's a crap load of people that wouldn't be meeting, falling in love, and having babies in America. Some people parents would never have been born, so you might never have been born.
Similar repercussions with baby boomers. So many babies born right after the war that most likely would not have been born, at least not as many. My mom was a post-war baby. The timing likely would have been different if my Grandpa wasn't on a ship in the Pacific during the war and I might not be here.
Then you have to take into account the economic impact of WWII. War economy and production catapulted the US into the forefront as industrial leaders of the world. The US would have recovered eventually from the Great Depression but it would not have been as quickly as the war allowed. Some might argue that a more gradual recovery may have been better, but the argument still stands.
Finally, you have to consider that the political climate in Europe was terrible with or without Hitler. Maybe some other leader rallies Germany. Likely without a mass genocide, but war may have occurred either way. Then there's Stalin. Before Hitler pulled a major **** up and decided to double cross Stalin, Russia was part of the Axis powers. The Nazis ended up being a nice buffer between Stalin and the rest of Europe. Hitlers ill fated attempts to attack Russia during winter took massive tolls in manpower and resources from both the Nazis and the Russians. Without Germany as a buffer the US and its allies may have been trying to fight the Russian juggernaut and Japan. Who knows what the outcome would have been in that fight.
So many different things to consider. Hitler was one man, but the his net effect on the world population was massive, both extreme negative swings and post war population growth. It would be incredibly interesting to find out what the results would be.