There's an old poker saying "you can't beat the rake". Two perfect players will eventually lose all their money to the house in rake or time charges.
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Zimmerman wasn't patrolling, he was running an errand, and when told that he didn't need to follow Martin he agreed and told the 911 operator he was returning to his vehicle.
Martin is still dead, though.
The "smoking gun" memo is most likely faked, see Megan McArdle's analysis at http://www.theatlantic.com/megan-mcardle/