You presume the shooter had a reason to confront the airsoft owner in the first place. All I read was the shooter thought he saw “the beginning if an armed robbery.” If that can be proved the case, that there was an ongoing crime, then the shooter is not guilty. If the shooter mistook the situation then he is likely guilty of manslaughter.
He wasn’t acting in self-defense.
Arguably he acted with willful disregard for the value of human life.
If the finder of fact (probably the jury) determines that the circumstances were not such that a reasonable person would believe that a crime was about to take place, then he loses his “justification” and a murder charge becomes feasible.