Hamlet hesitates to kill Claudius because he believes that Claudius is praying. If he were to be killed right after praying, when his soul is at its most pure, Claudius would go directly to heaven—and Hamlet wants to be sure he sends him to hell. But Hamlet can't hear what Claudius is actually saying; Claudius admits that he cannot fully repent of his murder of King Hamlet, because doing so would mean giving up his crown. So, technically, Claudius's soul is not pure, and Hamlet could have killed him in this moment, but Hamlet doesn't know this!