Very tragic for both the boy and the teacher really, that it escalated to this.
For what it's worth, I'll chuck my 2 pence in as a teacher.
I teach in a Ofsted 'Outstanding' school, although our grades aren't phenomenal and we have a higher than average kids on free school meals, with SEN etc. We have kids who are incredibly difficult, regularly isolated, put onto alternative courses - things like that. At worse though they are mildly intimidating, usually just loud, late, frustrating.
I think the kids in his class were not 'bad' kids, the ones that would be excluded or isolated. I think they were more likely to actually be quite bright - and it's those kids that are the worst when they choose not to behave. It's easy enough to coerce the nutters with a detention threat or a bit of shouting, but brighter kids laugh it off and have a strong group mentality. They can quickly round on a teacher, become incredibly intimidating and threatening and know all the buttons to press.
Fortunately, I've never experienced it, but know colleagues in the school who have and have taken time off with stress; they're perhaps victims of the subjects they teach or just lack the authority for it to escalate.
Judging by some of the more positive comments about the teacher, I don't imagine him to be a teacher who has ever lacked authority or the ability to mesmerise a class into working for him - that's certainly the easiest way of maintaining control. He was quite clearly ill.
However, knowing the worst kids in our school, I would never hit one. I'll admit to wanting to thump a couple of them in the gob on several occasions, not that it would actually solve anything. What he's done is inexcusable.