Artorias Syndrome
A physical condition caused by significant injuries to the left arm, which are left untreated, and have been exacerbated by frenetic physical activity. Can cause strong negative emotions, manifested as outright aggression, or excessive aggression.
Some dude broke his arm trying to defend a direct strike to his shield, but he never really did anything about it. Now he has Artorias Syndrome, and is always trying to start a fight because of the pain it causes.