Like I said, pure cowardice. He has people that depend on him, friends, family, relatives etc too. You're just being selfish.
Something many Americans don't seem to get is that billions of fights happen every week that end in nothing more than pain, bruises, a bit of bleeding or a broken bone or two at worst. You are probably as likely to die in a plane crash as you are to die from a fight you might get in to at some point in your life. The fear and selfishness motivates people to essentially take another human life at the meagre insignificant risk of danger.
There is no way TM was realistically going to kill GZ. And based on Zimmerman's injuries, he certainly did not fight it out long enough to see if he was close to any real danger.
And you don't lay there and take it, you fight back or you lose. Simple. If Zimmerman never racially profiled TM, and followed him AGAINST the wishes of the police and AGAINST the rules of his neighbourhood watch, TM would be alive today, and that's the bottom line.
That's why other properly developed Western nations around the world have laws that strictly prohibit killing people, even in the name of self defence. Because human life is important. The idea is that everyone should be aware of that, even at their limits, and having laws like those of most developed countries inhibits that.