In terms of Hamlet's philosophic view, these two "friends" from school were willing to take him to England and deliver Hamlet to his death, so what's good for the goose is good for the gander. He simply re-writes the letter, seals it with his ring which is an authentic seal of the King of Denmark considering he is a prince of the realm, and sends them to their well-deserved deaths.
I would also consider this a practical move, since Hamlet certainly does not want to die...at least not yet...as he has unfinished business to complete. He has been dispatched by his father's ghost to exact revenge on Claudius and any others behind King Hamlet's death.
As far as revenge goes on Hamlet's part, I don't see him vengeful toward Rosencrantz and Guildenstern. The play reads as though he considers them a stepping stone and a way out of a sticky situation. He doesn't waste time with remorse for them, but he doesn't seem out to get them either.
Today's moral standards? Hummm. That's more difficult since they differ greatly from person to person. Unfortunately, "right" and "wrong" don't seem to be so conveniently black and white anymore. As a student of literature, I tend to be able to see from all points of view. I can understand Hamlet's decision to redirect R&G so that he can surprise "daddy". Is it wrong to kill? Yes. Are there times when it's accepted? Sure...war being one of those times. Hamlet is at war.
Hamlets friends did not know what the letter contained, and had no opportunity to open it as it was sealed. They were simply carrying out a favor for the king, as any subject would do. Hamlet does not have a problem with them helping the king, his problem is with the way they try to manipulate him.