When I think about it, I think the fact that he had knowledge of what he was doing could be used to establish malicious intent.
Zimmerman knew he had a gun on him, but yet chose to enter into a confrontation. Who is to say he didn't defend himself on purpose just so he could kill someone and finally get the "bad guy"?
Reach, but it's definitely a point in this case and the precedents a not-guilty verdict could would be something for the jury to think about (ie. It's okay to establish conflict, lose, and then kill, all while knowing you had a deadly weapon on you). It could seem intentional under similar circumstances.
The precedent being: You can arm yourself with deadly weaponry and then knowingly engage in non-deadly confrontation and then use such weaponry to clean your ass.