] He had an opportunity to kill Claudius when he was praying, but didn't do so - "This is hire and salary, not revenge!" Ah, but hiding behind an arras, eavesdropping, sneaking around and spying as Claudius has always done, when Hamlet's emotions are on the edge and in the midst of a confrontation with his mother - that is a good time to do the deed!

The irony is, Hamlet had just left Claudius praying, so it couldn't be him behind the arras--but Hamlet clearly hopes it is. "Is it the king?" As if he hopes that fate has taken care of the deed for him.

As for killing Claudius while he is praying, it would be the perfect time. Hamlet forms his intent to wait until Claudius is at something "that hath no relish of salvation in't" and then strike. But what of Claudius' prayer? He's not going to change. He admits that he still has the objects for which he did the murder: "My crown, mine own ambition, and my queen" and his final words on the subject are:

My words fly up, my thoughts remain below;
Words without thoughts never to heaven go.

So Shakespeare was either having a very grim joke over Hamlet's thoughts on choosing the proper time to commit a murder, or else he might have been deriding the idea that one's actions at the moment of death determined one's eternal fate.