Brienne of Tarth turned out to be somewhat trustworthy in the end. A bit too idealistic and naive for his taste, but she was loyal and decent enough with a sword. Since she had sworn to Sansa, her loyalty was assured. Knowing that Ice, the ancestral Valyrian steel greatsword of House Stark, was melted and reforged into two blades for House Lannister made his blood boil. This was both an insult and a declaration. Declaration that House Stark is done for and defunct, and an insult to their ancient House.