"However the defense insisted he wasn't backing off, rather squaring up, taking a fighting stance, ready to whoop Drejka's ass. Watching the video again, this could be accurate."
Unless it's obvious to a reasonable person then that's a load of shit. Maybe he was getting ready to square off. Maybe he was getting ready to knit a wool sweater. Maybe he was getting ready to do a double back handspring with a full twist. We'll never know because he got shot before he did any of that.
If Drejka misperceived the situation, that isn't necessarily a show stopper. But the misperception has to be reasonable. For example, if the guy reached in his pocket after knocking Drejka down. That's a common prep movement for drawing a weapon. If that had happened and Drejka shot, I'd be defending him as much as I'm attacking him now, even if the guy was just reaching for his phone and not an actual weapon.
Taking a step backward is not a common prep movement to continuing an attack. Carrying a gun is a huge responsibility. Way, way, way more than most people realize. You can't just start blasting and say, "Well, I was kinda sure that the guy might have been getting ready to come at me..."
Drejka thought he was a bigger deal than he was. He wasn't the parking police. He had no duty to give them shit about their parking. And look what happened when he took that duty on; he created a situation that he couldn't control, he killed a guy, and now he's gonna be a convicted felon doing time in stir. Good work, sport. Real good work.