Board Thread:Questions and Answers/@comment-25371179-20160627111506/@comment-28895263-20160705032039

Prince Doran had said: "I believe in second chances.  I don't believe in third chances." That makes the motivation very clear.

Once Doran had learned of Myrcella's murder, he would have had no choice but to execute Ellaria Sand, and the Snakes. Trystane would have probably insisted on doing it himself:  He was truly in love with Myrcella.

After Ellaria & Co. killed Myrcella, there were only two paths. Either they kill Doran and Trystane, or we see Cersei opening four boxes, with four heads.