Comment: he's not on trial

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In reply to comment: have you forgotten it works (see in situ)

he's not on trial

Zimmerman is. It is not the defenses job to prove Martin was up to no good or was attempting robbery. It's been established by the prosecutions own witnesses that Zimmerman was being bashed in the head and therefore had a reasonable expectation to want to defend himself.

Zimmerman's guilt or innocence is the question here. And it is he that at the moment is presumed innocent until proven otherwise.