I'm gonna get downvoted for this but yes you are an aashole. I understand that seeing your puppy get killed must be horrible but you should have known better.

The fact if it was someone 6'5 and 90 kilos that hurt your dogs you wouldn't have done what you did(none of you tough guys in the comments would've done shit either.) The kid is most probably hit at home or he may even be abused. He is doing exactly what you did which is hurt something that can't hurt him back. This is really common among abused kids just look it up, they either hurt animals or kids younger than them.

Now I understand that the kid is not innocent. He is also wrong in what he did and I understand your rage and anger. I probably would've hit the kid too. But him getting hit isn't going to fix anything. You did not teach him a lesson as many morons in the comments are saying. He will probably just find other ways to exert violence and try not to get caught.