As long as you didn't provoke or invite the conflict, you have no duty to retreat from a life-threatening conflict. Legally you may stand your ground and kill the aggressor. (State v. Helm, 209 P.2d 187 (1990))In this case it is clear from the pictures and the audio track being played over and over that they boy was not the aggressor and that he was the one begging for help.
This clown could end up setting back gun rights but it the meanwhile he is bringing the race-pimp circus to town and damned if they don't have a reason this time.
If the tapes are being played over the radio, I bet they are being heavily edited.
This reminds me of the Rodney King incident all over again.
I do not see any incentive for the police to cover for Zimmerman. What do they possibly have to gain?