probably true. i would have done the same as you if it got to that point.
i think though the guy was in cop mode rather than security guard mode and the cop escalated this into a situation that he believed allowed him to shoot the guy.
If it is true that the guy charged with a hand in his pocket and yelling what he is alleged to have yelled, I think just about anyone would be justified in using lethal force under the circumstances, because there clearly was an implied threat of violence in his actions. Recall that Travyon was not armed yet was well on his way to killing Zimmerman after attacking him.
There are many uses of excessive force out there, but if the basic story holds up I don't think this is one of those.