Although Hamlet does not kill Rosencrantz and Guildenstern with his own two hands, he engineers their deaths by substituting a letter they carry with an order for their executions. The original letter called for the King of England to execute Hamlet on Denmark's behalf, but with the use of his father's signet ring, Hamlet is able to alter the commission and send Rosencrantz and Guildenstern in his stead.
Because Rosencrantz and Guildenstern proved themselves more concerned with Hamlet's treacherous uncle Claudius and attempted to shepherd Hamlet to his death, Hamlet feels perfectly justified in his manner of vengeance. He washes his hands of any guilt by explaining to his only remaining friend, Horatio, that "'Tis dangerous when the baser nature comes / Between the pass and fell incensed points / Of mighty opposites." Rosencrantz and Guildenstern, who allowed "baser" desires, like money, to overcome their friendship with Hamlet, have been reduced to casualties in the war between Hamlet and Claudius. Rosencrantz and Guildenstern's deaths also mark the escalation of Hamlet's moves against Claudius; once news of their executions reaches the king, he knows that Hamlet has reason to act against him directly and is on guard against any possible attacks.