Why does Shakespeare use the theme or motif of decay/rottenness/disease in Hamlet?
"Something is rotten in the state of Denmark," observes Marcellus in the fourth scene of Act I. In truth, the Danish court is corrupt. And, it is in the second scene of this first act that it becomes apparent that under the reign of Claudius Denmark is also weak because only by supplicating Norway does Claudius prevent Fortinbras from retaking lands lost by his father to King Hamlet. This corruption and weakness effect the decay and moral disease of the Danish court.
Weakness is demonstrated in the characters of Gertrude and Ophelia. In her weakness, Gertrude marries her brother-in-law rather quickly after the sudden death of her husband. She gives audience to the conspiratorial Polonius, believing him when he tells her that Hamlet is mad, and in cowardice she does not inform Hamlet that there are poisoned cups when her son duels Laertes in the final act. Although she does attempt to aid Hamlet, her actions are too late.
In her weakness Ophelia complies with the orders of her father to report what Hamlet says to her. Knowing that she is a pawn of Polonius, Hamlet toys with her, speaks disrespectfully and vehemently to her. Ophelia eventually breaks and commits suicide.
- Moral and Political Corruption
The villain of the play, Claudius poisons his own brother, then marries King Hamlet's wife in what Hamlet considers an incestuous relationship. He conspires with Polonius, Rosencrantz and Guildenstern, and exploits Laertes to rid himself of Hamlet by having the English kill him.
Polonius exploits his own daughter in order to implicate Hamlet as possessive of suspicious motives and mental illness. He lies about Hamlet to Queen Gertrude in order to further his own political desires.
Rosencrantz and Guildenstern, once school friends of Hamlet, agreed to spy upon their former classmate for King Claudius and are false when they talk with Hamlet.
Indeed, there is considerable evidence that there is a weakening of the state of Denmark and marked corruption. So, there is more than a little something rotten in the state of Denmark. Hamlet's first soliloquy is spoken about the dismal condition of the world, but they seem more prophetic of Denmark:
Fie on't! O fie! 'tis an unweeded garden
That grows to seed; things rank and gross in nature
Possess it merely.