If he believed that he was acting for the good of the world, then he would be forgiven. There is no proof that Hitler didn't go to heaven, because there's no proof that there is not a heaven. All that you need is belief. If there is nothing to believe in, then life is simply black and white, with a beginning and an end. There would be no purpose to life if there is nothing before it or after it.
I actually find the idea that "if I don't believe in an afterlife then my life is meaningless" borderline offensive. It also doesn't make any real sense. Basically there are 2 ways to look at it:
1) I believe in an eternal afterlife. Therefore, I have 100 years in this life and an eternity in another.
2) I don't believe in an eternal afterlife. Therefore, I have 100 years in this life and that's it.
It seems to me like when those 100 years are all you have, they become incredibly more precious. Lets just look at how these two attitudes translate.
1) Who cares about this life? After all, I have an eternally long one coming after, so why care about this one?
2) This life is all there is, so I'd better make sure I live it to the fullest.
I argue that the idea of an afterlife actually cheapens this one.