Personally, I support the conviction. Dunn made false allegations that one of the teens had pointed a shotgun at him. A shotgun is large and no reasonable person could make such an "error." IMO, this argument hinted at a calculated attempt to cover for what he had done following an incident that Dunn instigated. He also fled the scene and didn't report the incident. If someone killed in self-defense, he or she almost certainly would call the police so as to make clear what he or she had done and why. Therefore, his claim of self defense appeared not questionable but completely without merit.
Did he trigger the incident with the intent to kill? From The New York Times:
Prosecutors damaged his credibility by putting Rhonda Rouer, Mr. Dunn’s former fiancée, on the stand. In tearful testimony, Ms. Rouer, who was inside the store when the shooting took place, said Mr. Dunn complained about “thug” music. And in the night and day after the shooting, he never once mentioned that a teenager had pulled out a firearm.
IMO, the prosecutors demonstrated beyond reasonable doubt that Dunn had targeted people with the intent to kill. That he devised an excuse about seeing a shotgun, not a smaller handgun, at some point after he murdered the teen shows that it took him time to come up with a rationalization. There was no self-defense. Almost certainly, he came up with his rationalization only after he was arrested after fleeing the scene of his crime.
In sum, I believe the jury made exactly the right decision. This was a case of premeditated murder. He chose his victims on the basis of the music they were playing, triggered a confrontation, then fired at his victims, fled the scene, then invented an excuse after his arrest. He deserves life without parole for his crime.