The law as written is over reaching, but in all honesty to your question if you could make a decent case of why you "believed" at the time you were stopping a crime you very well could be immune in Florida for shooting that person in the back, even when you end up being wrong. As long as you can convince a judge first, or a jury second if that judge won't give you immunity, that it was a reasonable belief at the time.
I posted several recent cases in Florida that have been given immunity. All of them involved the shooter/stabber/living one to have pursued, initiated, confronted, and frequently lied thru their teeth. All were given immunity or acquitted. One was a stabbing that the stabber pursued for a city block over a car radio, not a child, a radio...... stabbed the man, left him to bleed out, took the radios, hid the weapon, went home and went to bed, no 911 call, denied the stabbing until he saw it was on video, then said the man was armed, then admitted okay he wasn't armed, but he swung the bag of radios at my head after I chased screaming like a knife wielding lunatic (with a prior criminal history), and that bag of radios the man was defending his life with could have killed me if they hit me.
Granted immunity at his stand your ground hearing, trial dismissed. There were several crazy ones, but that those are the rulings that will determine how GZs case has to be ruled, SYG hearing, jury or appeal..... somewhere along the way he will get standing. They didn't charge him that night because this path is inevitable. The law has to be rewritten, but GZ cannot be thrown to the wolves if this how the state has been handling the cases and told their citizens they interpret acceptable behavior. And if GZ has a permit, wants to play pretend cop, I promise you he is aware of the SYG rulings and they probably did play into his mindset.
He has called 911 fifty times, and followed several of them, no reports of confrontations. He probably felt pretty confident that night. But the law that night is the same one that let the stabber, several shootings, and one person who ran the other one over with their car (talk about an unbalanced fight and ability to flee) walk free. GZs case is much more arguable than those.