The punk as you describe him was on probation.
Just gotten out from an unspecified time in lockup. The victom never had a chance. Check the body language on the perp. No one home, the repeated return to assault his victom. Comes from being in lockup for a long time. No telegraphing intent like we socialised citizens do because we really don't want to fight.
In a lockup population you get to business if you are attacking someone. Other wise you are telling your apponent you are afraid. The audience was afraid. If any of them had helped it would have made no difference to the perp. He was already in auto attack mode. His brain was switched off, short of totally destroying his knee or shooting him, he may well have been able to take on the whole store. In the presence of that type of animalistic emotion emination, most civilized citizens freeze up.
Any statements of what you would do in the face of this kind of violence should be your private concern. In any training center for real world combat of this type you are forced to face and accept that these activities will probably kill you or make you a vegetable for life. That animal will give you only those options if you appose him.