Well, first of all, he was found not guilty and I think we should all move on. I'm no lawyer....but I know in my CCL classes (yes I'm a big-time gun rights and CC supporter) literally, the FIRST thing our instructor mentioned is having a CCL does not give you the right to "carry out free-lance police work". He was also told not to pursue the man my the 911 dispatcher and he did anyway.
We also learned in our CC classes not to use deadly force unless you see a life in danger. You are supposed to use equal force to the assailant. In other words, when you're a CC holder you can't go around starting shit with people thinking that you've got the "trump card" if things get out of hand. I think that's exactly what GZ did.....consciously or not.
Because of these things I believe he acted negligently and his actions contributed to the situation. I look at it like a drunk driver or a participant in a bar-fight killing someone and getting manslaughter. They don't set-out to kill someone, but their actions led to someone dying.
However, TM's actions contributed to the situation as well. No one forced him to physically confront Zimmerman. The whole thing is just a tragedy and I don't think there's a clear good-guy or bad-guy.