My cat is oft times aggressive and I had him neutered at 6 months old. So neutering doesn't necessarily fix behavior problems. We have had to give him one or two week "time outs" where we basically ignore him, no petting, holding, no lap time. It is so hard to do, but it does help calm him and reduce his agitation. Lately, it only takes a day or two of "time out" to bring him back his loving self. incidentally, this time out method was recommended by the Cat Whisperer.
Hate to think what he would be like if I didn't get him fixed.