Arren was hypnotized by Cob into killing his father.
In the movie, Arren is triggered into attacking the slave traders with a Slasher Smile on his face. When he is at Cob's castle, Cob tells him to attack Sparrowhawk, and he does so without question. This would explain why he doesn't know why he murdered his father in the beginning of the movie. Of course, that's just my opinion.- Cob showed interest in Arren and knew he needed Arren's true name to achieve immortality. I thought the implication was that he summoned Arren's shadow self in the first place.
