Well, basically the idea is this:
You believe in God, you receive the Holy Spirit.
The Holy Spirit, which is like God inside you, influences you--no, uh, let me say that differently.
If you accept God, it changes you. You become a better person. Therefore, anyone who goes to Heaven isn't like Hitler--at least not after they accepted the Holy Spirit.
Here's an example: A guy is sentenced to life in prison for murder. While in prison, he decides to accept God into his life. He changes for the better. When he dies, he goes to Heaven.
But if someone in the same situation except it's not life in prison says they accepted God, then gets released and goes off and kills someone else, then that means they didn't truly accept God, so they don't go to Heaven.