In act iii of hamlet, why did hamlet choose not to kill claudius when he was praying?

My favorite part of the play. The irony is unbelievable.
Everybody in the play is Catholic along with Shakespeare. If he kills Claudius now, Claudius will be in a state of grace which means that all of his sins (including the murder of Hamlet's father) will be forgiven and Claudius will be given a free ticket to heaven. That's the last thing Hamlet wants.

We learn that after Hamlet has made this decision, and leaves, Claudius tells that he cannot pray meaningfully. His words do not reach even the ceiling (if I remember correctly). 

Isn't that a bit of irony? Think of it. By the terms of the play, Hamlet thinks Claudius will go to heaven. But what Claudius tells us is that nothing could be further from the truth.