Part of what makes Claudius's crime of killing King Hamlet (Prince Hamlet's father) heinous is that Claudius murdered King Hamlet when the king was in a state of sin. He had not confessed; he had not had his last rites; he was simply snoozing. This explains why King Hamlet must walk the earth, for a period, as a ghost. Claudius not only killed King Hamlet's body, but blocked his soul's passage to heaven.
This information helps drive the plot of the play. For instance, Hamlet will not kill Claudius when he sees him at his prayers, because he believes that would not truly count as revenge: Claudius would die in a state of salvation and grace, unlike his father. Ironically, what Hamlet can't know, although the audience does, is that while Claudius looks like he is at prayer, in fact, he is thinking about how he simply can't repent of his sin of murdering King Hamlet. Therefore, Hamlet could have killed him then and justice would have been served. Instead, Hamlet waits and kills Polonius
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