Almost every death is caused because of someone's failure to play the Game of Thrones. I agree with fuckari, the Red Wedding was because Robb screwed over Frey. Frey considered himself to be the most powerful lord in the area, and to be slighted by a boy in such a way... betrayed and robbed of being father to a queen, yeah, he was going to kill Robb.
With the spoiler, he deserved to die. You don't demand a confession during something like that. You kill them, then consider the man guilty because the God's don't let a just man die. His pride was too great, and pride gets you killed in Westeros.
I loved Cersei's smile at the end. I can't wait to see her face when she remembers that Myrcella was sent to Dorne.