Hamlet admits to cruelty only when he is about to encounter his mother in the Closet scene, and then he seeks to qualify the term

The cruelty he seeks to permit himself is to be kept under a restraint, not let loose with the tyrannical savagery of which Nero served as a type. So again, at the end of the interview, Hamlet cries, 'I must be cruel only to be kind', claiming that his cruelty serves its opposite, kindness. What Hamlet seems anxious to do here is to prevent himself from inflicting cruelty for its own sake; and the fact that he alone articulates this idea in the play suggests both the measure of success he has in controlling himself, and also his awareness, so to speak, of possibilities for cruelty within himself.

If Hamlet is not at this point recalling the Ghost's speeches to him in act i, his concern about his mother, and the re-appearance of the Ghost in the Closet scene, make the link for spectator and reader. Then the Ghost had ended his account of the murder by exhorting Hamlet to revenge, but warning him too: