Hamlet seems especially cold and indifferent about sending Rosencrantz and Guildenstern off to their deaths. They did not really desereve to die, since they had no knowledge of the contents of the letters they were carrying to England and had done nothing to Hamlet except to try to find out why he had been behaving so strangely. They were not the only ones. Others included Gertrude, Polonius, and Ophelia.
Hamlet forges a letter and substitutes it for one asking the English to behead him. Instead he asks the English to behead the two messengers, Rosencrantz and Guildenstern. Was that necessary? Yes, because he felt he had to silence them immediately; otherwise they could have told the English authorities that King Claudius wanted Hamlet removed from Denmark, that Hamlet was insane, that he had just murdered Polonius, the King's chief advisor, and that the Danish king considered Hamlet a threat to his own life (which he was). This probably would have been sufficient to make the English detain all three men until they had a chance to communicate with Claudius. They might have sent Hamlet's forged letter to Denmark and asked if that expressed Claudius' true wishes.
Hamlet's life was in danger. He had to dispose of Rosencrantz and Guildenstern. At the time of writing the letter he didn't know that his ship would be attacked by pirates and that he personally would never get to England although Rosencrantz and Guildenstern would continue the journey and get their heads cut off.
Hamlet is still in danger, as Horatio reminids him.
It must be shortly known to him from England
What is the issue of the business there.
It will be short. The interim is mine.