I`d say he was concerned about his neighborhood and taking the gun with him was a great idea given the fact that baboons have robbed houses right in the very area where he routinely patrolled.
Trayvon`s choices created the situation and the outcome. The moron could have called the police or just kept walking, not paying Zimmerman any mind or could have gone to the nearest house for help if he felt like he needed it. What did he do instead? Confront and assault a man with a loaded gun.
Martins choice was to go walk to the store and back
Zimmermans choice was to arm himself and pursue someone after being told not to pursue
I'm sure we can agree that there is correlation between having a gun and shooting someone and not having a gun and not shooting someone
Zimmerman chose to carry a gun. He chose to follow that person (a threatening activity all by itself)
No one knows what went down after that but without a gun I'm pretty sure both Zimmerman and Martin would be alive right now....instead of one of them being dead
Zimmerman choices created that outcome