(continued, edit's being a bitch again.)
As you can see, the defendant George Zimmerman is actually required to prove quite a bit. As it was never established beyond reasonable doubt that he was even assaulted, none of these conditions are met.
A person is justified in using deadly force if he reasonably believes that such force is necessary to prevent imminent death or great bodily harm to himself...
...If in your consideration of the issue of self-defense you have a reasonable doubt on the question of whether George Zimmerman was justified in the use of deadly force, you should find George Zimmerman not guilty.
My opinion on the default presumption of justification in a homocide aside this is a pretty standard clarification of self defense. However, a very significant passage is missing here that is included in every other self-defense claim I've reviewed. (Emphasis added.)
First, the defendant must have actually believed that (specify) was using or was about to use deadly physical force against him/her [or someone else], and that the defendant’s own use of deadly physical force was necessary to defend himself/herself from it; and
Second, a “reasonable person” in the defendant’s position, knowing what the defendant knew and being in the same circumstances, would have had those same beliefs.
Thus, under our law of justification, it is not sufficient that the defendant honestly believed in his own mind that he was faced with defending himself/herself [or someone else] against the use or imminent use of deadly physical force. An honest belief, no matter how genuine or sincere, may yet be unreasonable.
The jury was prompted several times to consider the situation through Zimmerman's eyes and reasoning, but never was it suggested that they could disagree with it. This is especially crucial in a motion to lessen the charge to manslaughter.