I agree, you need to find him a good child pyschiatrist and/or psychologist. I too dont want to make you feel like something is wrong with him or something, but at the same time...that does not sound like normal behavoir.
My son was the same way. They are right that spanking wont help, my son quickly adapted and would laugh at me while I spanked him. All his violence started wellllll before any spanking though so I know that wasnt what caused it. Anyway, he was kicked out of daycare twice, I quit working so I could stay home with him instead, but everyday was hard. He would beat up on his little sisters, once he started kindergarten he had a very hard time there too. I'd watch him at the park walk up to random kids and punch them for no reason.
I had started him with a psychologist and psychiatrist when he was 5, about 6 months before K, because I knew it would be an issue. We tried other methods but nothing worked. Finally we put him on a form of Ritalin (didnt work), then Adderall (did work for awhile) and now hes on Vyvanse. We could see the difference in his ability to focus on tasks and whatnot but it didnt help with the violence. By this age, he was also having what we now call rages-basically violent temper tantrums resulting in holes in walls, his matress being torn to shreds, stabbing things, screaming and kicking for hours on end. He was placed in special ed in the middle of this year and has been only going to school 2 hours a day. I did some of my own research and read about bipolar in children and it described my child almost perfectly.
He started Depakote, at the miminum dose, about 4 months ago and even on that low dose the change has been dramatic. (Most kids are given Lithium, but my son has a history of epilepsy so it made more sense to at least try a siezure med first) He is only rarely having issues with hitting, and usually its more of a typical kid reaction, like when another kid was hitting him with a stick, he hit him back, that sort of thing. He hasnt had a rage since about 3 weeks into starting the med. He's doing very well at school and they are talking about reintigrating him into a normal classroom (Also, hes very smart, as most bipolar/adhd kids are-his IQ is in the 140's). I tried very hard to keep him off meds but the difference is amazing. He sees it too-he's only 8 but he will remind me that he hasnt had his meds and he has told me repeatedly he loves being able to sit still long enough to do things and feel like a normal kid.
Obviously I'm not saying this is what's up with your son, but I would DEFINATELY seek help and also be sure to do your own research as well. He's still too young for a diagnosis I believe, and I really wouldnt want to give meds to a child that young if I could avoid it, but simply seeing someone who can tell you no, you arnt a bad parent, this isnt your fault, is amazingly helpful and reassuring. Its even possible something else happened to cause this that you dont know about-as horrible as it is, I started having horrible violent behavoir when I was 3 and it turned out it was because I was being molested by a babysitter. Other things can trigger negative behavior like that too....Just get some help before you go nuts!