If he had merely exhibited even the slightest bit of body language that even remotely indicated that he perceived a continuing threat, I think the jury could have easily let him walk. Had he reentered the store and speeded up as though he saw the perp still moving, or backed up toward the counter to retrieve his second gun, or even spun around rapidly as though some movement or sound had alerted him and engaged from where he was standing...any one or combination of those cues or any number of others could have created enough reasonable doubt.
As it stands, his actions, all recorded on video, show that upon his return to the store, he did absolutely NOTHING to indicate that he was reacting to a threat or dange to himself or another person.
All the smoke and mirrors introduced on this thread have little to do, IMHO, with his guilt or innocence. The suggestion that he had previously been terrorized by gang bangers, his false claims of injuries and wartime service, his careless shooting down the street, etc. have little if anything to do with legality or illegality of his actions. They merely tell me that serious, responsible firearm owners, and proponents of legitimate self-defense need to pick our battles carefully, and this is not the hill to die on (no pun intended).