After you've killed the doppelgangers and saved the dukes in Sarevok's crowning seremony, the protagonist having said something along the lines of "Have a look at these documents"
Sarevok: "Angelo!!1! These are the men who killed the leaders of Iron throne, arrest them!"
Belt: "Wait! These men are not to be touched until I've read these documents."
Sarevok: "You don't need to read those documents, but since I know you're too stupid to take my advice, I'll just take your head!"
Just think about the anger he's going through while saying that! He'd seen how his plans were falling apart, piece by piece, and the final straw was struck when he was so very close to having his plans become true. One can't even imagine what he'd do to our main character if he got his hands on him. And how he acted afterwards; he killed his mentor, in his burst of anger, doing things purely by instincts, not by thinking things through as he used to, trying to save what there was left to be saved. The writers really managed to pick their words in that situation, not to mention the threatening music fitted perfectly