A Hamlet timeline – chronicle of events
Setting the Timeline – considerations
Claudius – planning my foul murder
King Hamlet's funeral – where was Hamlet?
Gertrude & Claudius – adultery or not?
Horatio – Hamlet's friend?
Horatio – is he passion's slave?
Polonius – the evil that men do
Ophelia's love? – did she love Hamlet?
Ophelia closetted – Polonius on love
O help xxx ....... – Olivier's version
Ophelia's change – is Hamlet suspicious?
Hamlet feigns madness – protective "cover"
Is Hamlet mad? – Polonius's opinion
Hamlet kills Polonius – stabs the "Voice"
Laertes on Ophelia – madness & death
Ophelia's death – a recipe
Hamlet's age – digging up the past
Yorick – something rotting in Denmark
Betting on Hamlet – the fencing match
Hamlet's fencing skill – better than Laertes
Hamlet kills Polonius
Why did Hamlet do it?
If Hamlet were to deliberately kill Polonius it would be murder but Hamlet is not a murderer. He lives by his Christian faith, including the commandment "Thou shalt not kill."
Claudius is a different case. He is a murderer and ought to be punished for his crime. The Ghost persuades Hamlet to not only execute Claudius but, imperatively, to revenge the murder by ensuring Claudius goes to hell rather than heaven. It must be clearly understood that Hamlet is not the initiator of this action but rather the Ghost who persuades him to do it. In modern parlance, Hamlet is persuaded to be the "hit man" of the Ghost.
When Hamlet has an opportunity to kill Claudius, he finds him kneeling in prayer. To kill him whilst praying, according to Hamlet's Christian beliefs, would sent Claudius directly to heaven and fail to attain the revenge the Ghost requires. Hamlet, therefore, defers killing Claudius until he can catch him in a sin that will guarantee to send him to hell:
'When he is drunk asleep, or in his rage,
Only a few minutes later, Hamlet stabs and kills the person behind the arras. Regardless of who Hamlet thought it might be, he never, for a moment, thought is was Claudius. Why not? Because if he had thought it was Claudius he most definitely would not have stabbed him simply because the act would have had no element of revenge. It was for that very reason, only minutes earlier, he had refrained from killing Claudius who was praying at the time. Merely killing Claudius was not the point, for to kill him would send him straight to heaven and that is not what the ghost had asked for. The ghost wanted revenge and for Claudius to end up in hell. Hamlet intended following that idea and stated the circumstances under which Claudius could be killed without hope of salvation. Neither Claudius being behind a curtain in his wife’s room, nor Claudius having just prayed, had any element of evil in it. Consequently, if Hamlet thought the 'voice' was Claudius he most certainly would not stab it, but because he did stab the 'voice' then, clearly, he did not believe it was Claudius.
Another reason why Hamlet never considering Claudius as the person behind the arras is a very practical one. While Claudius was still on his knees praying in the chapel, Hamlet could have killed him but instead left him and went straight to his mother's room. It was impractical for Claudius to get there before Hamlet did, and to conceal himself behind the arras.
So why did Hamlet stab the 'voice'? Quite likely it was self-defence. Given the dramatic ending of “The Mousetrap”, Hamlet would realise he had suddenly come under life-threatening danger from Claudius. Recall that Hamlet had feigned madness so that Claudius would consider him harmless. But, during the play he had completely blown his cover, and his life was in danger. Claudius had, indeed, planned to have Hamlet murdered. Hamlet, though aware of a potential threat, had no idea where, or when, an attempt on his life might take place. All he could do was be prepared and constantly alert.
Because Rosencrantz and Guildenstern were Claudius's men, Hamlet did not trust them. Guildenstern tells Hamlet his mother wants to see him. Then Polonius says the same thing. He doesn't trust him either, and it could be a trap. Hamlet deliberately delays and then goes armed with a sword and ready to meet assassination attempts on his life. Obviously, Claudius could easily have put assassins in hiding behind the arras in Gertrude's bedroom ready to leap out and kill Hamlet the moment they got a signal.
When he heard the voice from behind the arras call out "What, ho! help, help, help!", he may well have thought the assassin was about to strike and was calling on his fellow assassins to leap from hiding to kill him and his mother. His instant action, to protect his life and defend Gertrude, was to stab the voice.