We don't know, nor do we have reason to suspect, that Martin committed any crimes that night before his encounter with Zimmerman.
Apparently Zimmerman is claiming that Martin attacked him and that the shooting was self defense.
If Martin attacked Zimmerman then this was a crime.
Unless Martin himself was acting in self-defense. It's possible that both of them believed they were acting in self-defense. If it would be acceptable for one person to shoot another in self-defense, then it should be acceptable to fight physically in self-defense.
Again, I don't know what happened, either. But, the above is another possibility.