When you can not escape and the aggressor shows no signs of stopping. For example, if you were on the ground, and he was straddled over on top of you punching you.
Link
In the video, Zimmerman, with bandages at the back of his head, recalls being punched by the teen, trying to run away, calling for help, and then being grabbed and pushed down, his head being slammed against the cement sidewalk as his body lay on the grass.
As he tried to move his head off the cement, he says, he saw a neighbor emerge from one of the homes. He remembers calling for help, with the person responding that he or she would call 911.
He then says his jacket shifted upward, revealing his gun. When Martin saw it, he continued, the teen informed him “you’re going to die tonight,” and attempted to take the firearm. That, Zimmerman says, is when he reached for the gun himself, eventually shooting Martin.
He didn’t think he had hit the teen, and then heard him say, “You got me.” He moved the teen and spread his arms apart, looking to see if he’d struck him with something.
I don't exactly know what was told the jury so this may not apply in the jury room, but since Zimmerman doesn't seem to claim that he felt fear from the beating, but from Martin going for the gun I don't why I should factor in the beating or no signs of stopping in determining Zimmerman's fear. In fact doesn't Martin need to stop at least momentarily to make the move for the gun?
So my belief as to whether or not Zimmerman had sufficient fear is based entirely on how credible the above is and not other things. Since it is unimaginable (there are no reasonable ideas to create doubts) that the above is possible it all depends on what I must consider. If I have to consider whether the above is reasonable Zimmerman is guilty. If I have to consider whether Zimmerman believed the above unreasonable scenario then he walks.
The fact that other people (even me) think the beating alone meets the standard seems irrelevant to me. I need to have heard Zimmerman say it. But again maybe the jury was told a different story by the witnesses. (I have watched only summaries and a few youtube clips.) They were certainly told a different story in the opening and closing arguments. ETA: But that is not evidence.