He was not required to obey the instructions of the 911 dsipatcher. What Zimmerman did was not illegal, any more than the young man being in and walking around the neighborhood was illegal, where apparently family of his lived.
The key thing is the violence and altercation. Apparently Zimmerman gave up the chase and was walking back when he was attacked by this young man...who was getting the better of him and pummeling him.
At that point Zimmerman had every right to defend himself. Apparently, it was Zimmerman crying out for help on the tape.
If the young man initiated violence, it was illegal and irrespective of stand your ground, Zimmerman was entitled to defend himself. That decision to use violence was initiated by the young man...apparently.
But we have to let the facts come out and that is the real issue IMHO. Let the process work...we have a President and others immediately politicising it and convicting Zimmerman before he has a chance to defend himself or have due process.
There were recent photos all over here last night but do not post pictures.
So he says. According to Martin's girlfriend, Zimmerman started the confrontation.
You're right that Zimmerman did nothing illegal when he ignored the 911 dispatched and continued to follow Martin. It wasn't illegal, but it (and other aspects of the 911 call, in particular his comment to the effect that "these a**holes always get away") is evidence of Zimmerman's state of mind, and makes it seem plausible that Zimmerman started the confrontation, not Martin.