I don't think either of them broke the law and were both within their legal rights, Martin had a right to walk through the neighborhood and Zimmerman had a right to follow him.
The real issue is how the confrontation started but nobody seems to really know that for sure.
When I was a teen and thought I was tough $h** I would not have let Zimmerman follow me around and would have dis-respected his legal rights and tried to stop him with physical force if necessary. That seems like the likely scenario to me. However, I do know grown men who suffer from tough guy syndrome even at Zimmerman's age and it's totally possible he was the aggressor. If this is the case it would be unlikely Zimmerman would admit to getting his a$$ kick by Martin before pulling the gun. At least until he realized how much trouble he was in.