Hamlet’s primary theme is the moral ambiguity of action, played out in the quest for vengeance. Throughout the play, various characters seek revenge for various murders: Hamlet for the murder of his father, Laertes for the murder of Polonius, Fortinbras for the murder of his father. However, they take very different approaches to revenge. Because Hamlet’s quest is front and center, it is the one we learn most about; it is also the one that turns most heavily on questions of moral certainty. Hamlet continually postpones action because he does not have all the answers. First, he postpones action because he does not “know” that Claudius killed his father. Once he feels he has enough evidence to prove Claudius’s guilt, he postpones action again by musing that he cannot “know” what will happen to Claudius in the afterlife and thus he cannot “know” whether death will actually inflict the vengeance he seeks.
A secondary theme in Hamlet is the metaphor of the state as a diseased body. Denmark is repeatedly referred to as “rotten,” as a body in need of healing or as a patient in need of a doctor. This metaphor points to the internal corruption of Claudius’s court and of Claudius himself; it is significant that Claudius’s chosen method of killing is to introduce a corrupting agent (poison) into an otherwise healthy body (the king’s). The murder thus becomes a metaphor for Claudius’s takeover of Denmark itself.