I'm going with either
a. Rush Limbaugh, who put a contract out on him knowing it would draw national attention away from Slutgate.
or
b. Global warming denialists, who have delayed action on climate change resulting in it raining that night which caused Martin to pull his hoodie hood up thus giving him a suspicious appearance.
or
c. The High Fructose Corn Syrup lobby which keeps the addictive substance in candies such as Skittles, without which he would've probably stayed in that night eating rice cakes.
or
d. George Bush, who if he hadn't stolen the election, would have allowed us a President who would have done away with the internal combustion engine thus preventing Zimmerman from stalking people in his car.