The scene begins with Claudius tasking Rosencrantz and Guildenstern to take him to England. Then Claudius begins to pray over his “heavy burden.”  As he is praying, Hamlet happens by and decides quickly to stab him to death, but then realizes that he is shriving and will go to heaven, unlike his father who was killed “unhousled…unaneled” (1.5.84) and has to burn off sins in hell. Hamlet will wait until he kills Claudius while “…drunk asleep, or in his rage…about some act that has no relish in it” so “his heels may kick at heaven”(3.4.94-99) as he plunges headfirst to hell.

Yes: Hamlet would drink hot blood right then, but decides to wait to assure not only revenge, but the justice of Claudius in hell.