Since Hitler was in a state of mortal sin, he would have to confess to a priest and be absolved of those sins. A simple act of contrition would not apply, that would be for venial sins. Also Hitler would not go to Heaven even if he had made a confession. He would go to Purgatory. Who knows for how long?
Further, we can't know for certain the state of Hitler's soul. There are three things that must be simultaneously present to commit a mortal sin, and we cannot know whether or not they were present in Hitler's mind. Only God knows, and He knows perfectly. They are: 1) the sinner must know with "moral certainty" that the action is seriously sinful; 2) they must give full consent; and 3) sufficient reflection. Sometimes these are stated as 1) serious matter; 2) full knowledge; and 3) full consent. The term "moral certainty" does not mean mathematical certainty. Rather, "moral certainty" means that he must know with sufficient certainty to make a moral decision.
Whichever way you phrase them, together the three requirements mean the same things. One of the consequences of this is that they mean that an insane individual is not morally responsible for their actions if their insanity is not their own fault, and if it did not allow them to fully appreciate the gravity of the sin, or to give full human consent to it. We cannot know, with certainty, what Hitler's state of mind or soul was when he died.
Assuming Hitler was not insane, and that all the three conditions for mortal sin were simultaneously met, then we can say that if he made an Act of Perfect Contrition before he died, then his ultimate destination would have been heaven. Of course, as Catholics, we believe that he almost certainly would have a long period in purgatory before going to heaven.
As with my other comment on an Act of Perfect Contrition, I will not be drawn into an argument about this. Either accept it or don't. I'm willing to clarify what the belief is, but I'm not going to get drawn into an argument with Protestants about it.