] 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.

But Hamlet knows none of this. The stage directions indicate that he doesn't enter until after Claudius has said his "O, my offence is rank" speech. All Hamlet sees is him kneeling and praying. Claudius doesn't say "My words fly up, my thoughts remain below: Words without thoughts never to heaven go." until after Hamlet exits.

Yes, it's ironic that it would have been the perfect time, for Claudius is not really seeking forgiveness, for he knows he doesn't deserve it, but Hamlet has no way of knowing this. And I think he does believe that an unprepared soul, one which has not sought forgiveness, an unshriven soul, suffers more after death than one which is.

The Ghost told Hamlet as much, it is one of the grievances against Claudius.

"No reckoning made, but sent to my account
With all my imperfections on my head:
O, horrible! O, horrible! most horrible!"