I agree. Let's say this metally handicapped individual had the intellectual capacity of a 5 year old (a concervative estimate, as a 5 year old would probably understand the hostility directed at him).
Now what would you do if a five year old peeked up the skirt of a little girl? Bang his head against the window and then beat the crap out of him? I am sure the mother and the girl felt unfomfortable by the situation, but you are basically dealing with an individual who does not comprehend the implications of what he is doing.
The correct response would have been to alert the bus driver, who would have arranged for the police to come and pick the guy up. Then the mother & child, the two violent retards and the rest of the passengers would have been able to continue their journey.......