Why are Trayvon & Zimmerman Held to Different Standards of Justice?

Here is the actual jury instruction read to Florida juries prior to the legislature's enactment of Stand Your Ground.

"The defendant cannot justify the use of force likely to cause death or great bodily harm unless he used every reasonable means within his power and consistent with his own safety to avoid the danger before resorting to that force.
The fact that the defendant was wrongfully attacked cannot justify his use of force likely to cause death or great bodily harm if by retreating he could have avoided the need to use that force."