Hamlet has turned his faith from reason to action. What is problematic about a complete turn from one to the other? How successful do you think he has been in divorcing himself from his intellect? Does really even try?
He goes from thinking everything through too thoroughly to not thinking anything through. Hamlet stops considering consequences, and from that point on, no one in the play is really safe. Everyone dies the moment he decides to put his bloody thoughts into action. He is one of the most cerebral characters in literature, but in Act V, he doesn't do much thinking...it's Gertrude, Laertes, Claudius, and Hamlet, one after the other...biting the dust.