From the instructions
In deciding whether George Zimmerman was justified in the use of deadly force, you must judge him by the circumstances by which he was surrounded at the time the force was used. The danger facing George Zimmerman need not have been actual; however, to justify the use of deadly force, the appearance of danger must have been so real that a reasonably cautious and prudent person under the same circumstances would have believed that the danger could be avoided only through the use of that force. Based upon appearances, George Zimmerman must have actually believed that the danger was real.
If George Zimmerman was not engaged in an unlawful activity and was attacked in anyplace where he had a right to be, he had no duty to retreat and had the right to stand his ground and meet force with force, including deadly force if he reasonably believed that it was necessary to do so to prevent death or great bodily harm to himself or another or to prevent the commission of a forcible felony.
http://mediamatters.org/blog/2013/07/15/stand-your-ground-and-the-zimmerman-defense/194870
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