Peltiers' innocence had been debunked by other Indians.
I am always amused by the way Peltier's supporters argue, simultaneously, that Peltier wasn't even there and had nothing to do with the murders, and that Peltier killed the agents in the middle of a quasi-military battle, and is thus not guilty because he was defending himself.
Pick one argument or the other. But it is hard to see how standing over two fallen agents who have both been shot multiple times and executing them at point blank range while they beg for mercy can be called "self defense".