So Robin Williams is in hell?

No, I don't think Robin is in hell. He wasn't of sound mind when he died. People with his illness have reported hallucinations and similar problems. In my opinion, God wouldn't condemn him for a physical illness that influenced his mental processes. In many ways, the physical problem that caused his death is similar to other physical problems that cause death. God wouldn't condemn someone that died of cancer and I don't believe he would condemn Robin either - especially because (from all accounts) he was a particularly wonderful person.
Rest In Peace Robin and feel the love that so many have for you.

This was explained in the movie when Chris was wanting to find Annie. The 'Hell' that she was in was created by her. Just like he created his world. The emotional turmoil she was in when she committed suicide created a world for her that is dark and full of pain and she can't make it out to see beyond that pain.

No one knows what happens to us after death but I love this movie and the fact that it depicts many views of the afterlife all rolled into one. As far as Robin, I hope he has found peace and isn't stuck in his own Hell.

I hope not

He's in limbo, according to Catholic teachings. We have to remember that he was part of the normalisation of homosexuality in the 90s with the movie The Bird Cage.