Evidently, children who fought less were improving their skills at empathy and using those skills in other contexts. Some siblings have very different temperaments. These kids tended to fight more often unless the older child's temperament was easygoing. You are right that a child who feels angry should not hit someone, and so forth, but there are times when emotions are important cues that should not be dismissed. You would want him to pay attention to those feelings.
Feelings are a weakness.
You have to be tough to make it in life.