The condo HOA settled, Zimmerman was acting under community authorization, watch for precedent to be set on this basis.
He was not acting under law or statute. That is the qualifying element for a federal prosecution. The fact that he was a neighborhood watch guy in the community provides no standing. He was enforcing no law, nor did he have the power to arrest, detain, stop, or search. There may be, will be, civil suits against him but there will be no federal charges.