Zimmerman acted stupidly and even disobeyed police orders to stay in his car. I also have trouble believing that TM would fight an adult unless Zimmerman did something to provoke the situation (beyond simply questioning him). And the fact that he brought his gun with him says something about his intent to me. Not only are these actions ill-advised, but they resulted in the loss of a human life. If he can't be convicted of anything, I actually see it a shame. You seem happy about it, which is strange to me.
There ought to be a punishment for disobeying cops where the end result is the loss of life.