My personal feeling is that Zimmerman simply should have followed better procedural protocols. He was not trained to chase, follow, or apprehend a subject. His job is to serve as the "eyes and ears" for the neighborhood. He even admitted that he failed to introduce himself. Now, Martin probably should have contacted the police rather than the individuals at hos home. But think about when you were a teenager in your own neighborhood. For most individuals in that age group, in your own neighborhood, your first attempt at help would likely be your parent, guardian, or other individual at home. Police will not be able to respond as quickly as they are not right there, whereas your parent or guardian is.
There are many points in this scenario that could have gone differently. Where Martin was in the past week or days was irrelevant. In my eyes, one obvious mistake was made: Zimmerman did not properly introduce yourself. Yet security personnel are trained specifically to present themselves before following someone. Especially when security personnel is not in uniform or any other obvious form of clothing that makes it clear to others.
I know Zimmerman isn't white and further, I don't believe Zimmerman is racist. But his actions were not completely justified. There was no "street", and the events that occurred aren't known to us or anyone else except for Zimmerman himself. The end fact is usually when one is killed with a firearm, the one responsible is apprehended while investigations go underway. That particular police department didn't do that. They took his story and without any further questions, let him go. Several parties were wrong but if Zimmerman were afraid for his life, why follow him?
As I said, I don't believe Zimmerman should serve time for being racist, but he should have gotten something for killing a young man. Perhaps you don't have children or a loved one and thus don't completely understand. But there is a family out there now that must eat dinner at night with an empty seat. They go to bed at night knowing their son is gone forever and they didn't even get to say farewell.
Think about peacefully going to bed tonight and waking up the next morning to find out the a close family member of yours is now deceased.