We can't know what was in his mind. I think it more likely that his perception of himself as the "protector" of the neighborhood against evil people, esp black men, made him feel he was empowered to stop and control anyone he believed suspicious. If the suspect put up any resistance or refused to submit, deadly force was an allowable option.
A subtle distinction, though, and certainly one that is irrelevant to the victim.
It's interesting to note that since Zimmerman has not been arrested or charged, he currently has the legal right to carry a gun and continue his "neighborhood watch" activities if he wishes to.
Yeah, I like your theory better than my original idea.