I think you've gotten a lot of good responses here. I just wanted to give some input that you mightn't have seen.
When I was about 9, I started 'acting up'. This was mostly in the form of not always doing homework, having (what I now know as normal) conflict with my sister and not always telling the truth when asked. (Much of this was due to my anger at my mother's having had an affair, resulting in the breakup of two families, new stepsiblings, and a new baby brother.) These aren't the same issues as what you are dealing with, but let me explain-- my mother and stepfather began what became a full-scale campaign of changing my behavior through punishment. This would have some devastating consequences for me in the coming years.
Punishment and rote isolation can damage a child's sense of self. It takes a long, long time to repair these damages. It sounds as though your son is really furious and is actually being honest about his anger. That, in itself, is a good thing. I'd give him safe outlets to express himself: paper and markers, playdough, let him crash cars or trains if he needs to, let him punch pillows and then, let him tell you what he needs to without fear of judgment--when he's ready. He might be feeling that he's pretty unacceptable in the opinion of others, and may even be doing some pre-emptory acting out, rejecting others before they can reject him.
Counseling for both himself, and your family, can help. I'm so glad you have already started down this path. It will be a worthwhile investment in everyone's happiness, because I'm sure this impacts everyone.
As for Halloween, I'd be thoughtful about taking this away. I wouldn't use it as a threat or privilege, per se, but just wait and see how things go. If he's acting out in the days immediately before Halloween, then you can explain it to him as a logical consequence -- "We see that you are really having a hard time and that you are hitting other children. We have to be responsible for keeping the other kids safe, too, and so this time, you'll need to stay home. All the kids have the right to feel safe on Halloween." This become less of a punishment and more a consideration of others in relation to his actions.
My heart goes out to you. It sounds like you really love and care for your son, and kids can be such a mystery. The techniques in both Positive Discipline and the parenting classic "How to Talk So Kids Will Listen...And How to Listen So Kids Will Talk" can help immensely in getting discipline on a more responsive and proactive track (punishment, in my opinion, is largely enforced reactively) and will also improve the communication within the family.
I don't know if he's too old for this, but Mr Rogers has some great dvd's entitled "Adventures in Friendship" and "What do you do with the mad that you Feel?" which give a lot of support without judgment. I personally am still not too old to appreciate Mr Rogers, and his empathetic approach to hard, conflicted feelings can still be helpful/instructive at this age.
My heart goes out to you and your son. You are lucky enough to still be in a place to mend hurt feelings and hurt hearts, and hopefully, to move forward together. Good for you for getting help now. My best to you.