I disagree about corporal punishment. What we are really talking about, if it were between adults, is pain oriented compliance. We have no idea how this 6 year old got to that point, but I suggest if he had been subjected to more 'creative' forms of correction we may not be talking about a violent reaction.
Somewhere the along the line, the cycle of violence has to be broken. I came from a very violent atmosphere, surrounded by alcoholics and beset with an abusive older brother. The Strap was the method of both teaching and punishment, only the nuns never spanked on the ass, they just used a really big ruler. The only way I can think of how to break that cycle is being an example the best I can. With my wife's help, we raised two wonderful daughters who were never touched.
As a society, Canada is learning that jail is a very poor form of correcting a wrong. I believe we need keep in mind that righting a wrong is the objective, and that physical punishment does nothing toward that end.
Given your background, I'd be surprised if you weren't opposed to corporeal punishment.
I grew up in an Ozzie and Harriet, Father Knows Best, Leave it To Beaver home, 1950's idyllic. Well, more or less. My parents were teetotalers and if they weren't perfect, by gawd they weren't too far from it. They were quite strict and did not tolerate disobedience or back talk, and yes they used corporeal punishment sometimes, but they were also very loving, kind, generous, cheerful, happy, and mostly reasonable.
They just expected their kids to behave and do as they were told, and enforced this as necessary... but never in an abusive or excessive manner.
I raised my son in a very similar fashion.
So we come at this from two very different perspectives, two very different backgrounds.
Now to clarify my position, I do not espouse spanking as a general or common punishment. IMO it should be rare, and reserved for major offenses or times when nothing else is working. Safety issues (child runs into the street without looking), or open defiance, yes. Passive defiance of a repetitive nature (ie been told three times, still haven't complied despite warning) in some cases.
But not as a general or common form of discipline. Some children are relatively easy to raise and never require it. Others are nearly impossible to manage if not spanked occasionally. It depends.
Crossing the line into abuse is always wrong, though. IMHO the big metal ruler across the knuckles (or face) was abusive and should not be allowed; and even when applied to the buttocks one should use something flexible and "stingy" that will not leave lasting bruises. The object is to make a lesson/order memorable by associating disobedience with pain, and a reminder that bad behavior has unpleasant consequences. As Nota noted, you can't reason with a toddler, and it is better that they suffer a stinging sensation about the heiney than get run over by a car, no?
But to each his own; I understand you have your reasons for your position.