From the initial 911 call, it's pretty clear that Travon was running away, with Zimmerman following him. The comment by Zimmerman that "These assholes always get away" sums up his frame of mind. He wasn't going to let the kid get away. If Zimmerman had just stopped following the kid, no one would have gotten hurt. Instead, Zimmerman initiated a confrontation. So why would Travon not be justified in using self defense to protect himself from someone chasing him?