Another important point here is that Hamlet's willingness to send his "friends" to their death shows that Hamlet is, in fact, capable of action. He is doing what is necessary here. Upon hearing what Hamlet did, Horatio says "Why, what a king is this!" He is congratulating Hamlet on acting in a kingly fashion by taking out those who are not loyal to his throne. While Rosencrantz and Guildenstern may not know the full extent of what they were getting into, they clearly know that they put their allegiance with the king and not their friend, and we as readers kind of cheer on, like Horatio, the man who has had so much trouble acting in the first 4 acts of the play.
For the audience, it is hard to feel that they deserved their fate: they were just two guys, taking orders from a King, trying to help their pal.
However, for Hamlet, who had felt deeply betrayed by all who were close to him (his mother, Opehlia and Polonius, and finally his two close friends), their deceit was just another in the line-up, and most importantly, a final testament to his uncle's guilt. Through their letter, he finally has rock-solid proof, and can now act instead of moping and groaning about the circumstances. So their death, to him, was most likely just one step in a series of necessary ones in order to accomplish his mission: avenge his father.
This suggests a singleness of purpose, how deeply he felt betrayed, and Hamlet's fatal determination to right the wrongs that had occurred, no matter who died.
Rozencrantz and Guildenstern are involved in the covert actions that swirl around Hamlet. Although they were once friends of Hamlet, they are now more concerned about "kissing up" to the the king to maintain/improve their standing in the court and are willing to carry his instructions to England to dispatch Hamlet. There is delicious irony in their arriving with notes they think will take care of Hamlet, only to be taken to their own deaths.
Do they deserve this fate? I don't know if they deserve it, but I certainly do not feel much sympathy for them. After all, they were willing to assist in Hamlet's death....
I think Hamlet's lack of remorse tells us how deeply depressed he is and how "all things do conspire against [him]."
There is a certain poetic justice in that they were dealt with in the same way that they intended to deal with Hamlet. There is the strong-running theme of "He who sets a snare will fall in it."
Furthermore, in setting up Hamlet to be killed, they were betraying a friend. There was no loyalty to the state here but pure self-interest. Hamlet turned the tables on them in self-defense. He did it without second thoughts - to save his own skin.
I agree with Cadena.
Hamlet was feeling very betrayed by all of his friends and family, his two close friends went behind his back to help the king, the same king who killed Hamlets father. So Hamlet himself is of course going to feel upset, he was betrayed.