To the point they said there, I think Gaige would have had a self defense/ defense of others argument if he had killed Kyle. Rosenbaum was responisble for all the chaos because he couldn't resist the urge to chase a minor. Not even a gun could deter his crazed pedophile yearnings. Gaige could have gotten away with being a dope who thought he could play hero, even if he guessed wrong.
I think some cases have turned out that way where a third party who gets in the middle of a deadly fight, but makes the wrong call doesn't go to jail because they don't have a guilty state of mind. They were trying to help, they were just exceptional.
Of course, his feigning surrender and being a sneaky bitch about things could have counted against him. Kyle was very open and honest in his self defense, and that was in his favor.