He was told to backdown an let the police handle it. he didn't.
Zimmerman, speaking in his first media interview since the Feb 26 killing of Trayvon Martin was asked to explain his statement to a 911 dispatcher that he was following Martin, Zimmerman said, "I didn't mean that I was actually pursuing him." "I meant that I was going in the same direction as him, to keep an eye on him so that I could tell the police where he was going," he told Fox News host Sean Hannity.
The encounter began when Zimmerman called police to report what he called "a real suspicious guy" walking through his neighborhood. In an affidavit submitted in court, two investigators assigned to the case by a Florida special prosecutor reported that Zimmerman "disregarded the police dispatcher" who instructed him not to follow Martin.
Zimmerman said after he got out of his car he never went further than 100 feet (30 meters) from it. He said he wanted to find out Martin's location to pass along a more accurate address to police.