George Zimmerman was not properly doing his job as a volunteer neighborhood watch captain. He had no right to follow a person and make contact with the person. If you suspect something while performing that role, you call 911 give location and stand way back, and you should not carry a gun on watch, even with a permit.
How do I know this? I lived in Florida for 18 years. I was in a gated community and pulled neighborhood watch duty many times. I also had a concealed gun permit for 17 years in Florida, which is not easy to get. You get a book of laws for when you can use it. I guess George Zimmerman did not understand the laws or just did not pay attention.