The thing is, he really had no reason to stop Vicious. Julia was dead, so the rivalry was over. Vicious was the new leader of the Red Dragons, a crminal organization. He wasn't planning to take over the world or blow up the galaxy. Once he took care of the "old guard", he probably would have even called off the orders of eliminating Spike and friends. Essentially, Spike was free, but he couldn't let go. He had to face Vicious. "Find out if he was really alive", as he put it. Everything that tied him to the past was gone. He had a future, with Faye and Jet, even Ed, once she had done some "soul searching". But he chose to go down in a blaze of glory. For nothing.