This sounds like a frivolous post but it's really not.
Even without the gun you couldn't go up and slap the kid. It would still be assault since he had not done anything to physically endanger another person. You would still be looking at an arrest record and possibly a criminal record.
You are not allowed to be a street avenger at any time.
Here's my credo: There are no good guns, There are no bad guns. A gun in the hands of a bad man is a bad thing. Any gun in the hands of a good man is no threat to anyone, except bad people.