] Just a thought, for some reason, I never really thought it was what Claudius may be saying in his prayer, but the fact that he was praying at all that stopped Hamlet.

That's true--but Hamlet assumes that Claudius really is praying, and he's not. If Hamlet had known that Claudius was not repentent and not actually asking for or receiving forgiveness, he wouldn't have had the worry about Claudius going to heaven, and the story would have taken a different turn.