Zimmerman had a legitimate reason to be curious about Martin. Martin took offense to the scrutiny because Martin was a racist punk. His offense took the form it usually does with racist punks: he attacked the so-called offender. In the process, the "offender" shot him.
If -- and it's a big "if" -- Zimmerman was a racist, it did not take the form of him attacking Martin. Martin's racism was blatant, and it -- along with an inflated sense of his own status -- got him killed.