I am amazed that more teachers don't snap.
From what I hear (my dad is a maths teacher in a rough school) some children can push you to the edge. Threatening you, hurling abuse at you, sneering at you, spitting at you, swearing at you, refusing to work, being backed up be "don't you say anything to my child" parents who think it is ok for their child to throw a chair at you, but not ok for you to push their child away from you to prevent them from punching you in the face?
Of course it is totally unacceptable to bash a kid round the head, of course it is, but can you imagine what a person must have endured for their mind to snap and for them to do something as terrible as that to a child?
And of course a reply is "If you can't handle it, get out of the profession" and while that is an option - how would our children be educated? Because if every teacher who had to face unacceptable behaviours just quit... there'd be nobody left. And then what?
Something needs to change. Some kids are out of control and teachers have no authority or power and the parents of the worst kids tend to be the least supportive of teachers' efforts to keep discipline.
He should not have hurt that child. But he should not have been in a position where he was driven to the edge.