I agree it was self defense. For the sake of argument however, if he reached his car to retrieve his weapon, did he have the option to leave the fracas instead of shooting?
It reads like he got his gun to defend himself AND go back to defend his friends. Even though he had his gun out, one frat thug still came at him. That indicates to me justification. If it was him alone and he could have driven off but decided not to, which is NOT how I understand what happen, that could be seen differently.
Even if he had the option of leaving, he would have left his friends were down on the ground, defending themselves from a larger mob, who had already attacked him (sucker punch is a pejorative term to me indicating no provocation for the attack). He may of have the OPTION to flee, but no such duty - and in fact, had a DUTY to help his friends, who were under attack.
The question is - did these guys do something BEFORE being physically assaulted/attacked by the fraternity brothers? (Did they saw, steal some wallets, beat up someone, rape a sorority sister?)