Yeah... there's that too.
A hypothetical time traveler could have to reckon with the rippling cascading effects of his actions. Killing someone BEFORE he or she became evil, makes them innocent victims and AS SUCH, and not as evildoers, they could be remembered afterwards.
This without minding the fact that the situation in the late 30ies WOULD have fallen in war even with no Hitler about.
Funky time traveling mechanics... until it'll happen for real we can only speculate.