In the last scene of Shakespeare's Hamlet, the only survivor is Horatio. He alone has been privy to all that has occurred in the kingdom since King Hamlet appeared on the battlements in Act 1 and told the truth of his death to his son. To everyone else, even Hamlet's mother, Hamlet has put on an "antic disposition," which has made everyone believe he is insane.
Hamlet's unexplainable behavior has been for those he does not trust: Claudius (his uncle, and his father's murderer), Polonius (his uncle's "lap dog"), Gertrude (the mother who seems to have betrayed his father's memory and is involved in an incestuous relationship), and Ophelia (who serves not her sweetheart, but her King and father).
At the end of the play, as Hamlet is dying, he asks Horatio to tell his tale. What Horatio refers to in his speech with Fortinbras summarizes everything that has taken place in this final scene:
So shall you hear
Of carnal, bloody and unnatural acts;
Of accidental judgments, casual slaughters;
Of deaths put on by cunning and forced cause... (395-398)
The "carnal, bloody and unnatural acts" would refer to Claudius's murder of Old Hamlet and his marriage to Gertrude. The Elizabethan audience believed that when a man and woman married, they became as one person. When Old Hamlet dies, some essence of him remains in Gertrude. When Gertrude sleeps with Claudius, the Elizabethans would believe that Claudius was sleeping with the essence of his brother, thus committing incest. Hamlet says to Claudius at the end:
Here, thou incestuous, murderous, damned Dane,
Drink off this potion! (332-333)
"Accidental judgments" would most likely refer to Laertes's desire for revenge for Hamlet's accidental murder of Polonius.
Exchange forgiveness with me, noble Hamlet.
Mine and my father's death come not upon thee... (337-338)
"...casual slaughters" refers to the death of Gertrude. There seems to be no desire on Claudius's part to kill the Queen, but when she starts to drink the poisoned wine—intended for Hamlet—he virtually does nothing. Although he could have knocked the drink away, he does not, for then his treachery would be known and his plan foiled:
The Queen carouses to thy fortune, Hamlet. (292)
Gertrude, do not drink. (294)
I will, my lord; I pray you pardon me. (295)
The last part of the quote is:
Of deaths put on by cunning and forced cause... (398)
This refers to the deaths of all in that hall, all caused by Claudius through his treacherous planning.
Horatio shares this news with a sorrowful Fortinbras, who will obtain the Danish throne, as Hamlet wished with his dying breath.