Everyone agrees that a small bird could be severely maimed by a smack.
The culprit knows this as well as anyone ~ and feels terrible about these events. It was a knee-jerk reaction ~ and I'm not sure that we can prevent those, when we are suddenly shocked.
Now a lesson has been learned and, thankfully, the bird seems ok.
I don't think that there is any need to allow this thread to descend into arguments & unpleasantness. It won't help, or change, the situation, if this happens.