All kinds of things involved in the Martin case, but here's how I think things went down:
Zimmerman, a guy with previous anger management issues, likes throwing his weight around, and thinks he's A Big Deal doing community watch. He's not a racist, but being raised in a racist society, he profiles a black kid in his neighborhood. Not being too bright, being a little overzealous, and ignoring the dispatcher's warnings, he decides to take matters into his own hands and play a tough guy and go after Martin anyway, even though Martin has started walking/running away from him.
Pursued, and possibly confronted, Martin is probably scared and defensive - after all, he's just walking home and this weirdo starts following him. He reacts with his own words, and possibly hands (although the latest info seems to contradict this part of Zimmerman's story).
An altercation happens, and things get out of hand - Zimmerman shoots, probably not out of malice, but also probably not by accident (I'd lay odds he thought just brandishing the gun would stop the fight). Knowing what to say (having already been through this before), the cops show up and the first thing Zimmerman says is "self defense." The cops do their cursory duty without looking too deeply into things and let him go.
Issues involved are complex and could be looked at separately, despite their interrelational natures in this particular case - societal racism, profiling, Zimmerman's history of aggressive problems, Florida's fucked up gun laws, this particular town's fucked up police department, and what we, as a society, are going to do about all of them.
The whole thing is just fucking sad, no matter which way you slice it.