It seems another intuitive not taking into account the details of the case.
I'm not talking about general streetfights or society or moral implications at all.
What I'm talking about, is that Martin could have potentially killed Zimmerman. You've done dumb stuff, right? You have friends who broke others' noses, right? This was MUCH more than a simple broken nose. Zimmerman was on his back, screaming for help 14 TIMES. He obviously didn't want to have to resolve this through force, but when you're having your skull bashed in repeatedly, it's sort of tough to think clearly. The use of force for Zimmerman to defend himself was justified.
If he's never been in a real fight before, then all the more reason for him to carry a gun to protect himself. That sort of defeats any argument you had for manslaughter. Zimmerman was looking out for society, and considering Martin's brutal attack on him, almost killing him and not stopping, (who stays within 18 inches of someone after you've broken their nose, and then push their head into the concrete?), Zimmerman did a great thing for society, as Martin could have used his anger that night to attack someone else.
Martin is the real 'murderer' if you can say that in this case, or at least a loose cannon. I've been 17 years old too. I've been impulsive, and I have done dumb things. I even got into a fight with a kid once in junior high, I will tell you, and I pushed his head into the ground. But I backed off then, almost going into a fist fight. I didn't break his nose or continue to smash his head. AND I knew the kid too. I don't fly off the handle to a complete stranger and almost kill them.