My child saw his uncle almost kill some one how do i explain to him how wrong that is?

Recently during time with my family my brother was in an altercation with a neighbor which resulted with him stabbing his neighbor. During the fight my son cheered for his uncle and was happy that he defeated the other person. When i got to my son he excitedly told me exactly what happened. I was mortified and immediately told him that what happened was wrong and that he was never allowed at that house anymore. During the next few days he again shared the story with two other people and was very happy to tell every detail of th story. I need to stress to him how wrong the situation is and that violence is never the answer. I need some real advice and if needed some information on child counseling.