The more I think about it, the less I am convinced that we have enough motivation for the change in Hamlet. Someone I watched Branagh's film with referred to the difference as "Hamlet in khakis" -- until then, Branagh wears only black, but when he comes back, he's in this buckskin-like breeches-like costume. (Cheryl, calm yourself =>;])

The person who is frustrated and angry enough to stick a sword into an arras and kill someone just doesn't seem to be in evidence anymore. The scene at Ophelia's grave is very rash--but he's confronting her death, the "maimed rites" tell him it was a suicide, and he's being accused of causing her death -- and all at once.

That's a whole bunch of nastiness inside a minute or two, and the fact that he gets really angry is a sign to me that he is much, much more confortable with himself than he's been since the action started. He defends himself, and he hasn't really done that through most of the play, when he's really upset, he calls himself names (like rogue and peasant slave) or talks about the shame he feels in not having revenged his father yet.