What does Hamlet think of Rosencrantz and Guildenstern?
That depends on when Hamlet might be thinking of them. The reader is told that the two characters are friends of Hamlet, most likely the same way that Horatio is friends with Hamlet. So at one point Hamlet likes these two guys.
The problem though is that Rosencrantz and Guildenstern abuse their friendship with Hamlet. At least that is how Hamlet interprets it. At the point in the play when Rosencrantz and Guildenstern show up, Hamlet has been acting crazy. Claudius (Hamlet's enemy) has asked the two men to spy on Hamlet to discover the source of his insanity. Hamlet sees right through the two of them and feels betrayed. Instead of being true to Hamlet, they are working for "the bad guy."
Hamlet discovers a letter that Rosencrantz and Guildenstern were carrying that ordered the death of Hamlet. That basically severed all ties with them right then. He reworked the letter so that it ordered the deaths of his two previous friends. It's possible Rosencrantz and Guildenstern didn't know the contents of the letter, but that didn't matter to Hamlet. They spied on him for Claudius and acted as his messenger with a death note. Hamlet sees them as traitors that deserve to die.