I dont think you went to far. There is absolutely nothing I would not do to protect my loved ones, my babies. I have a huge phobia of spiders/scorpions/snakes, but hey all of a sudden for the first time in my life, I killed the crap out of a spider, why? Stupid bug was coming after my baby. (I checked the wall after I went into my the spider must die rage, and there was a hole in in it the size of a grapefruit.)
We all have that instinct to protect our loved ones. You didnt do anything wrong. Maybe you feel bad because he is a special needs person, but dont, you shouldnt, this is not your fault. His mother is doing him a huge disservice by not training him that his behavior is not acceptable.