Did Trayvon Martin have a rap sheet? I remember some people putting the wrong picture of another kid that was more "thuggish" but I don't remember anything about a wrap sheet.
Either way, while I agree that most/all of the people in these situations have criminal histories, that doesn't mean they should have been killed. Things like resisting arrest, shouldn't result in being shot. There are cases like with Brown where I feel the officer was likely justified, and there are cases like (South Carolina?) where a guy is shot running away, or (Nebraska?) where the volunteer accidentally shot and killed a guy instead of going for his taser (having held both, you really shouldn't make that mistake, that guy is a freaking moron)...
I don't think it's an officer problem, I think it's a training problem. There are (mostly) good guys that are put in these situations and they make the wrong call. Honestly I feel that having a high number of combat veterans is adding to this problem, and I say this as a combat veteran. I see from a lot of friends who have gone into the police force, there's a high number of vets who carry the same attitude as a police officer that they did as a grunt. It's better to be judged by 12 than carried by 6, it's us versus them. While I definitely would rather a police officer go home to his family than a criminal, when you add that to the "us vs them" mentality it can be dangerous.
An us vs them attitude is good when in the military, your job is to defeat and destroy the enemy either through direct combat or support of direct combat troops. It is you vs the enemy. With police though, your job is to serve and protect the community. Us vs them isn't good. Us vs them makes you shoot quicker, and shoot when you shouldn't shoot.
Better training to help guys break away from the us vs them mentality. We shouldn't expect criminals to be smart, but we shouldn't lower the standards for our officers. It's a tricky situation for sure, cops need to be able to protect themselves, but we can't have people, even criminals, being killed over petty crimes.