why does hamlet stop himself from killing Claudius in this scene? what is ironic about hamlet's decision?

Hamlet doesn't kill Claudius because he is praying. Hamlet doesn't want to send the king to Heaven absolved of his sins especially when Claudius had killed Hamlet Sr. while all his sins were upon his head. There is irony her. Hamlet has the chance to kill Claudius and he doesn't. Hamlet wants to be a man of action but loses himself in metaphysical debate rather than just doing the job.