There are a number of prominent themes of William Shakespeare’s Hamlet. Each of the key themes we have identified in the following article, though, throws out some surprising details and interpretations, so it’s worth probing some of the play’s most important themes and subjects in more detail.
Revenge.
Revenge is obviously an important theme in Hamlet, if not the most important theme of all. Indeed, Hamlet is not just a tragedy but an example of a ‘revenge tragedy’, a subgenre of play which was already established when Shakespeare wrote Hamlet. Shakespeare’s own dramatising of the Hamlet story was itself a reworking of an earlier (sadly lost) play, probably by Thomas Kyd, which had been performed around a decade before. Only two words of dialogue of this Ur-Hamlet have survived: ‘Hamlet, revenge!’ Kyd’s other plays included The Spanish Tragedy, which, as we’ve discussed here, helped to make this kind of play popular on the Elizabethan stage.
Hamlet is told by the Ghost to ‘Revenge his [i.e., King Hamlet’s] foul and most unnatural murder.’ The Ghost identifies the King’s own brother, Claudius, as the murderer, and Hamlet Junior swears to avenge his father’s death. But as Hamlet will later confide in a soliloquy, he struggles to act and spends more time contemplating his revenge than he does actually carrying it out:
Why, what an ass am I! This is most brave,
That I, the son of a dear father murder’d,
Prompted to my revenge by heaven and hell,
Must, like a whore, unpack my heart with words,
And fall a-cursing, like a very drab.
Ultimately, of course, Hamlet does succeed in avenging his father’s murder, when he stabs Claudius with the poisoned sword and forces him to drink from the poisoned chalice for good measure. But in a sense, by this point his actions are as much a revenge-killing for his own murder (Claudius poisoned Laertes’ sword used in combat with Hamlet), so the motive behind the act is more complex.
Of course, Laertes conspired with Claudius to use a poisoned sword during his duel with Hamlet in order to carry out his own revenge: he wants Hamlet dead for killing his father, Polonius, and driving his sister, Ophelia, mad following his callous treatment of her. So Laertes, like Hamlet, is a son bent on revenge for the murder of his father. However, a third character, the Norwegian prince Fortinbras, is out to avenge the death of his father, who was killed by King Hamlet.
Delay.
Hamlet does technically get his revenge, ultimately, but only at the end of the play. He spends four acts of this long 
tragedy putting off his task: the killing of Claudius. Hamlet’s delaying tactics, his inability to carry out vengeance, is one of the most famous themes of Hamlet. He seems incapable of acting on his promise to avenge his father’s murder.
But does Hamlet even delay? Technically, we might answer ‘yes’, but in another sense, there are good reasons for his caution. First, as we mention below, Hamlet cannot be sure the Ghost is who it says it is. It claims to be his father, but there’s always a chance the Ghost is some demon sent to trick Hamlet into killing an innocent man.
He needs to establish that Claudius really is guilty. He hears Claudius confessing to his ‘offence’ while at prayer, and only fails to kill his uncle then because he doesn’t want Claudius to be sent straight to heaven. He puts on the play, The Murder of Gonzago, in order to trick Claudius into revealing his guilt. And he even stabs Polonius, who’s hiding behind the arras, convinced that Polonius is Claudius. Clearly, as soon as he’s gathered the evidence that Claudius is guilty as the Ghost claims he is, Hamlet is prepared to carry out his revenge. But then he stabs the wrong man and is then sent away by Claudius before he has a chance to kill him. Indeed, Hamlet talks about not being capable of action, but his actions themselves show him to be more decisive than he lets on in his soliloquies.
Kingship.
One of the uncomfortable truths about Hamlet is that Claudius may well be a better king than his predecessor. Of course, this wouldn’t make it right for him to have ousted his brother in order to seize the throne for himself, but it may nevertheless be the case that Denmark is better off with Claudius on the throne than King Hamlet. After all, King Hamlet killed Old Fortinbras, prompting Prince Fortinbras, his son, to invade Denmark in revenge; and we know (because he tells Hamlet this himself) that he was known to spend the afternoons asleep in his orchard (where he was murdered by Claudius) rather than attending to the affairs of state.
Another question which we are invited to ponder is this: when a king dies, in most countries the crown automatically passes to his oldest son, if he has one. When King Hamlet died, why did Hamlet not become king? Why is Claudius on the throne at all? The play does not broach this issue, but it is one of many intriguing puzzles about it. Was Hamlet deemed not fit to be king? Did Claudius insist?
Family.
The very first line Hamlet utters in Hamlet is (probably) an aside, muttered under his breath, in response to his uncle’s overly familiar greeting: ‘A little more than kin, and less than kind’. Family is clearly an important theme in Hamlet, from Hamlet’s duty to avenge his father, his fraught relationship with his uncle (even before he learns Claudius was probably the one responsible for his father’s death), and, most of all, his curious bond with his mother.
Does Hamlet’s anger at his mother’s hasty re-marrying stem purely from Hamlet’s loyalty to his father, or is Hamlet a mother’s son who has never quite cut the apron strings? And that’s to say nothing of Sigmund Freud’s Oedipal interpretation of their relationship.
Death and the afterlife.
Hamlet’s most famous speech in the play, his ‘To be, or not to be’ soliloquy, is one of the great meditations on what happens when we die. His other most famous line in the play is probably ‘Alas, poor Yorick’: words delivered over the skull of the dead jester, beginning one of the most famous meditations on mortality in the English language.
These things are especially on Hamlet’s mind because of the task he must carry out. What if the Ghost is not really his dead father, but has merely assumed the appearance of King Hamlet so it can trick him into performing evil deeds? This depends in part on one’s Christian beliefs: the Ghost announces that it has come from Purgatory, which is a specifically Catholic doctrine: it’s where souls go to be purged of their sins before being sent on to heaven (or, if the sins are too vast, to hell).
The appearance of the Ghost has prompted Hamlet to consider what will happen to him if he murders Claudius and Claudius turns out to be innocent. His own soul will be in jeopardy. These are big questions for Hamlet to weigh up, and so it’s hardly a surprise that he ‘delays’ his revenge initially, while he endeavours to discover the truth.