His mistake was getting out of the truck. If he was afraid that Martin was indeed armed, he should have backed away. Pulled back and waited for the cops. Even if you are armed, close quarters in the dark after you lose sight of the guy, that is dangerous, as Zimmerman now knows.
I wouldn`t have gotten out of the truck, but that is a judgement call. Getting out of the truck doesn`t mean his intention was to confront, that he did in fact confront, or that he initiated any kind of violence.