Board Thread:Questions and Answers/@comment-25371179-20160627111506/@comment-26271908-20160627152044

Actually, the point of killing the two was never the alliance. They killed them as vengeance because the Sand Snakes blamed Oberyn's death on the Lannisters. Doran was against a war so he was killed while Trystane would assume after his father so he was killed too. An alliance with Daenerys was never the point. It was purely emotional