The problem with the law is that too many people don't understand it, including some of the cops. You have the right to shoot someone if they are threatening to you. That does not include chasing them down, comforting them and then when THEY attempt to defend themselves, you then kill them
Private guards and cops have uniforms and marked vehicles for identification. Neighborhood watch groups do not. They are also not licensed but mainly, they do not have the right to approach you, ask you questions, follow you and certainly not chase you down. Zimmerman did not have that right with Travyon Martin and he does not have that right with you or any other citizen.