Not Guilty was the correct verdict. Whether you agree with him getting out of his car and following Trayvon or not, it wasn't against the law. The only time a law was broken in the entire scenario was when Trayvon Martin attacked George Zimmerman and beat him up.
You can accuse George Zimmerman of bad judgement but not of breaking any laws. He only used his gun when he felt his life was in danger, and rightfully so.