In the play's own words, Hamlet gives reasons for the delay and, at another moment, admits that he does not know why he has not yet acted.
The first is the doubt about the ghost. In Act II, scene 2, he says that the phantom "may be the devil," that the devil can take "a pleasing shape," and that he needs "grounds more relative than this." He then stages The Mousetrap, and only after the king's reaction does he tell Horatio that he takes the ghost's word.
The second is the scene of the prayer, in Act III, scene 3. Hamlet finds Claudius on his knees, raises his sword and desists, because killing a man at prayer would send him to heaven: "Oh, this is hire and salary, not revenge." He does not hear what the king says right after he leaves: "my words fly up, my thoughts remain below." Shortly afterward, in the queen's chamber, he kills Polonius, thinking it is the king.
In Act IV, scene 4, on seeing Fortinbras's army, Hamlet wonders why he still lives to say "this is yet to do," since he has "motive, will, strength, and means," and suspects that the cause may be "some cowardly scruple of thinking too precisely on the event." Yet in Act V, scene 2, with the proof of the royal dispatch, he answers Horatio that being ready is all.