There is a difference between approaching someone in broad daylight to ask a question and following/chasing someone at night through the rain. The first is civil, the second is belligerent and provocative and would scare the "average man" for his safety.
If you believe the story the kid ran away, Zimmerman broke off the chase and headed back to his truck. Then the kid came back and attacked him. Unfortunately and tragically the boy was killed. A terrible incident but no chargeable offense. Thats why the police have not arrested Zimmerman.