That is one of the great paradox questions.
Most people would assume that had you assassinated Hitler prior to WW2, that you would go down in history as the greatest hero of all time who saved tens of millions of lives.
But the far likelier reality is that you would go down in history as an evil villain - just another assassin of a world leader.
So instead of being known as the man who prevented the Holocaust and WW2, you'd be lumped in with the John Wilkes Booths and Harvey Oswalds of the world.