1. Zimmerman said he looked like he was on drugs, so it would take away one of the points that Zimmerman was profiling Martin. He also said "they always get away with it before even stopping him
2. Martin may have become paranoid due to the THC, prompting him to confront Zimmerman. Of course, that paranoia may have made him timid.Allegedly the THC was in minute traces. It stays in your system for a month.
3. Some apparently believe THC can make someone aggressive. In my experience that aggressiveness would be limited to competing for munchies.