A lot of insightful things have been said about the Trayvon Martin story, and I don't feel much need to duplicate them here. The tragic intersection of culturally manufactured fear with our deep commitment to racial narratives and the valuing of property over persons... it's a recipe for disaster, to say the least.
I don't by any means think all cops are corrupt and power hungry. I don't by any means think that there are no good cops. What I do think is that the mindset and culture of most police agencies is such that it attracts a certain type of personality and encourages a certain type of behavior. I have jokingly said to friends before "there's two kinds of cops: those who were bullies as kids, and those who were bullied." While I was joking, and I do think there are a few cops for whom this isn't true, generally speaking, this is a good rule of thumb to bear in mind when interacting with a cop. You should be asking yourself "What does s/he have to prove? What personal insecurity or childhood scar is this particular badge and gun supposed to be compensating for?" It will make you both more cautious and more compassionate when interacting with cops.
However, this only works if you happen to be in a privileged class, in terms of law enforcement. If you seem suspicious in any way, or you don't conform to the (generally very socially conservative) norms that most cops utilize to make their judgements about those with whom they interact, this probably won't help you. Your best bet is to avoid cops altogether. Like by not walking alone in your own neighborhood at night with your hood on, or being young and male and African American to begin with.
...back to Zimmerman, though. So the details that are emerging about George Zimmerman reveal a person who was one of these types of cops to the core. Every person he interacted with who wasn't clearly a part of some law enforcement agency or wealthy and white was a suspect. Any individual he fixated on was guilty until proven innocent, and he would never allow some paltry little facts to get in his way. He walked around looking for a fight, and some means of soothing/boosting his ego. His view of himself as a "good guy" was so central to his identity that it drove him to irrational and sinister extremes.
This Zimmerman guy - he's scary. At this point, a lot of Americans are realizing how scary he is. But what's more scary is that hundreds of thousands of others who are remarkably similar to him in profound ways patrol the streets and prisons of America every day. People who's ego needs salving, who find reassurance of their strength only by using the constructs of the police and prison system as a weapon against others whom they have personally deemed to be of little or no value to society. The fact that our law enforcement and penal systems attract and encourage these types? That is a serious problem that we face as a nation and that will continue to produce collateral damage in the form of broken bodies and ruined lives.