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.
WMG / Tales from Earthsea