You would have to deal with the social, cultural, and economic factors which lead to the rise of a person like him to power. Simply killing one person in the grand calculus of the social factors at play would likely do very little to stop WWII and it’s atrocities. Changing him could have lead to a more radical or desensitized power hierarchy that could have been much worse.

Preempt the war by taking back info on the atomic bomb to Roosevelt so that it could be utilized earlier and more effectively as a deterrent.