Let's look at the stats:
Trayvon got suspended for 10 days from school in South Florida.
He went up to Sanford to stay with his dad.
During his time there a rash of break-ins occurred.
Zimmerman being a member of the Neighborhood Watch,was alerted of the break-ins and instructed to be on the look out for suspicious activities and naturally,strangers.
Trayvon was walking in the rain.
Zimmerman spotted him and called 911.
Zimmerman was told to end pursue,which he did.
On his way back from his vehicle,Trayvon confronted Zimmerman knocked him down and proceeded with the physical assault of Zimmerman.
Zimmerman's voice calling for help is the one on the tape,not Trayvon's.
Zimmerman shot him in self defense.
Why did the police allow Zimmerman to walk free that night?
Simple,witness account and the injures he had were consistent with his story.
Why was he then later arrested?
Because the race peddlers got involved.