several witnesses have stated in interviews that they head the KID screaming for help, only to be corrected by officers that "no, you heard Zimmerman screaming" The police dept has a long history of biased investigations, so yeah, the public NEEDS to be on top of this to make sure that a thorough and unbiased investigation is completed.
The police have said that he is not being charged under the stand your ground law, while at the same time the legal experts (including the drafters of the law) say that he is not covered under it. Why? Because he put himself into that situation! He got out of his car, went chasing after someone even though he had reported it to the cops and been told they were coming.
Now, in fairness, Martin wasn't the small slight kid who is shown in most of the pictures. He is over 6' tall and about 200 lbs.
Thaiturkey, yes it can be legal, depending on the situation. You will likely be charged and the courts will sort it out. Your lawyer will then have to show that you had reason to be in fear for your life (jumped from behind, threats that they had a weapon, etc) It will then depend on the jury's opinion of whether of not an average person would feel threatened in that situation.
In this situation, there were no witnesses just standing around. People were in their homes, heard the screams and gunshot, then went outside to investigate.