"But you can't indulge in wanton violence. Where is your empathy?" growled Eragon, pointing at the head. (47.39)
The head here belongs to the slaver Torkenbrand. Murtagh removed it from his body, although Torkenbrand was defenseless at the time. Murtagh sees this is a totally justified act, but Eragon is on the other side of the fence. His point is that, justified or not, raw power without empathy (an understanding of one's fellow man) is not just. This drives a wedge between Eragon and his fighting buddy. Whose position makes more sense to you?