The Zimmerman case was unusual in that there were so many other sources of information, even if not for the last few moments before he fired the gun.
When the policeman lied to Zimmerman and said that it had all been captured on video, Zimmerman was relieved believing that he would be cleared.
“By your logic, no one would ever get off on a self-defense plea unless there was an impartial third-party eyewitness.”
By your logic anyone could say they were threatened with deadly force and use that to justify killing someone. Suspects lie, or misperceive things in the heat of the moment so there has to be evidence other than a hearsay threat. In this particular case there was physical evidence of an attack sufficient to support death or great bodily injury. People who are familiar with self defense laws know what keywords to use in statements to the police. Zim was clearly justified in his actions but sometimes people put some frosting on the cake just to be sure they wont be charged.