They have a statement from an eyewitness saying Tray was on top of the Zimmerman beating on him. I'm guessing that's why they believe he was defending himself
And there's no evidence that he WAS defending himself from an attack.
But there is a dead kid and plenty of circumstantial evidence for an arrest to launch a full investigation in order to determine guilt or innocence.
That's how the system works.
Really? What law allows people to be arrested and kept locked up with no actual evidence they committed a crime?