What exactly would be wrong with a game based on Hitler? -Even playing as Hitler? Yes, it would offend certain people who may have been affected by his actions, but there would be nothing inherently immoral or wrong about such a game, given that morality is nothing but relative subject judgments and is not absolute.
I'm frankly quite sick and tired of war games where you always play as the American fighting some sort of "evil" foreign threat ending with American victory. Of course it is done that way because it is primarily Americans playing these games, but I think a game from, for example the Iraqi perspective where you play as a freedom fighter driving out the American forces from the country would make a very interesting and fresh title as well.
And certainly playing as Hitler would make a very interesting game as well, perhaps if you "win" the game would go on to show the effects of your actions, the scale of human destruction and depravity (which would be far greater if he had won) -making for a quite thought provoking climax. Or perhaps giving you the possibility to enact your own sick policies afterwards. This is slightly off topic, but I'm tired of game companies playing it safe and adhering to some non existent standard f morality.