well, if i spilled my pop on him (because i wasnt looking or he was in my way, either way...), calls me an asshole and shoves me, for a fistfight to ensue, i guess it would mean i would attack him for shoving me...correct? i mean if he shoves me, he's probably not going to be coming after me for a follow up.
in that case, if i came back at him, i *could* be considered the agressor because i had an option to walk away. IF i was found to be the agressor, i can not pull my gun, legally
now, if i spilled my pop, he shoved me, and was crazy enough to pull out a pocket knife and say he was going to kill me and started coming at me (extreme end of the spectrum, i know), at that point my gun would be drawn letting him know to rethink his decision. if he still came at me to the point where i felt there was no stopping it, he would get shot.