The subject of Hamlet's letters underscores the motifs of illusion versus reality, seeming versus being, and pretense versus genuiness: Having witnessed the fortitude of Fortinbras, "the delicate and tender prince" who is willing to go into battle and risk his life "even for an eggshell," Hamlet decides that he, too, will act as a prince. So, in Act IV, scene 6, Hamlet sends Horatio a letter from England telling him that his ship was overtaken by pirates; he says that his ship put up a good fight, but after he boarded the pirates ship to fight, they pulled away from Hamlet's ship and he was taken as the only prisoner. Because the supposed pirates have not killed him, Hamlet says that he must repay them with "a good turn." So, Hamlet asks Horatio to take the letters he sends to the king. Then, when Hamlet writes that Rosencrantz and Guildenstern are still going to England and that he has news that will make Horatio speechless, some guise on Hamlet's part is evident. Apparently, Hamlet has arranged for the demise of his two treacherous friends.
When a messenger brings letters for the king and queen from Hamlet, Claudius is perplexed; he reads that Hamlet is returning to Denmark "naked," meaning with no possessions. And, Hamlet pointedly adds "alone," implying that he has eluded Rosencrantz and Guildenstern who have been told to kill Hamlet. Then, Hamlet asks the king for an audience at which time he will recount the "reason" for his sudden return. This recounting may take the form of physical action.
At this news Claudius is alarmed and solicits Laertes to duel with Hamlet and kill him. Since he seeks revenge for the death of his father, Polonius, Laertes fails to realize the pretense of Claudius, who is worried about Laertes challenging him for the crown, as well as the threat that Hamlet now presents. For, at this point Hamlet has decided to act upon his resolve of Act IV scene 4 in which he vows,
O, from this time forth, My thoughts be bloody, or be nothing worth! (4.4. 65-66).