I don't think he was being an ass, I think he was really juiced up and ready for more. It must send the adrynaline flowing to hear all those thousands of people screaming.
I thought him waving him up was pretty badass. Margarito, for some of the faults he has, is an absolute juggernaut. I honestly believe Williams simply had the style to keep Margarito at bay for 12. And even at that, I thought Margarito had Williams in trouble towards the end of that fight.