Hi Squirt,
You sound just like I felt not so long ago. We'd never had anything like that before and it was devastating. I'd start crying just telling people about it.
It seems to be behind us now, but I'm still not sure what caused it: I suspect it may have been a combination of things.
Like you I went back to smacking, which I hadn't done in ages. I went through all sorts of angst after the first time I did it, but came to the conclusion that in this instance it was necessary. He was violent to us, was damaging the house and putting himself in danger. So it became a regular response to certain kinds of behaviour, and although I felt awful doing it I really think it helped. I think that having a consistent and immediate consequence perhaps gave him more faith in me as a maintainer of limits. It would always be followed by lots of affectionate cuddling, confirmation of love, confirmation of expected behaviour, talking through of emotions. We often couldn't talk to him at all until spanking - his face would glaze over and he would scream to cover up our voices if we tried to speak.
Other things that may have helped... A lot more physical affection - even more than usual, because we're quite keen on that anyway. His teacher called us at home a couple of times to talk through what was happening - I think he appreciated the concern. I changed my parenting approach to give him more responsibility for managing himself: he has seemed to respond really well to that, and the plus side for me is much less nagging! Should have done this ages ago. I also tried to talk him through issues I think he may have been having with his school group - to confirm with him that it's fine not to play with people if you don't like the games they play or the way they behave to you. And that if you like playing with the girls then that's fine too. I arranged some play with non-school friends to take the focus away from the school group, particularly with his besty from kindy who, now I think about these things, is probably GT too - we didn't appreciate how lucky we were at the time. Walks in the forest have always helped to restore his equilbrium. And more adult conversations - eg about the relative merits of systems of government, whys and wherefores of terrorism (he likes contemplating political/moral issues) - engage him with us and I think help him in the process of sorting out his view of the world and his place in it. Sorry - that last isn't put very well at all - hope you can salvage some sense from it.
I sincerely hope your troubles don't last too long - it's an awful place to be.
Regards
BK