You underestimate the man's stupidity. First he calls the cops, before engaging in his hunt for prey. Then, after catching the kid and shoving him around, Zimmerman, while temporarily at a physical disadvantage, called for help , but only if you believe the eyewitness. Sybrina Fuller says that Martin was calling for help, ostensibly while Zimmerman held him at gunpoint. Anyway, Zimmerman is so stupid, that after shooting Martin, he stayed on the scene, and cooperated with police over the course of the next two or three days, giving them statements and answering their questions.
If that doesn't scream out "GUILT!", I don't know what does.
You forgot the part where he lied down on the ground and banged his head on the concrete.