Let me try to understand his logic. He must be over 18 and was going to tear into an 8 year old because the 8 year old was going to tear into a 2 year old? That’s really teaching the 8 year old good behaivor! Maybe witnessing a pattern of behavior like your son’s is part of the 8 year olds cause and effect response.
An answer to your question is that you treat the 8 year old no differently than a non-cah child. If you would have punished a non-CAH child, then punish the CAH child. When they grow-up thinking of themsleves as victims --for whatever reason-- (lack of disipline, holding them back from normal activites, etc.) they BECOME victims. So, treat him normally and don’t give CAH a second thought. What you witnessed is typical kid behavior (except for your son). I can almost guarantee you he wasn’t going to hurt the 2 year old but rather comminicating how badly he hurt. (hasn’t hurt anyone yet has he?)