Idiot #1 “He said he chose a random person from the other group to shoot at as that person was running away, the affidavit says.”
If the affidavit is true, this dude is going to have zero credible claim to self defense. It is an affirmative defense based on reasonableness. Having admitted to picking a “random person” who was also running away, its not likely he’ll be able to argue self defense.
Idiot #2 “When investigators told Miller they had video of him chasing someone in Mays’ group and shooting, Miller admitted to firing four to five shots, the affidavit said.”
Looks like they have video evidence he also gave pursuit. There are possible scenarios in which someone could give chase to someone who has already shot them and still claim self-defense, but its exceedingly rare that this would be successful in court. Generally you have to be able to articulate a reasonable and imminent threat, and as a private citizen in public you are unlikely to succeed on this claim if the person is running away from you.