re: Crackpot ASOIAF Theories SPOILERS

Theory time: Darkstar works for Littlefinger. Littlefinger likes keeping swords very close to all of the big power players whenever possible. Wanting to cause as much destruction in the realm as possible, he knew putting a hired gun near Myrcella and having that gun kill her would almost for the Martells and Lannisters to go to war with one another, especially with Cersei in charge. He is back on his way to the Vale to report to Littlefinger.