Sorry, “THC in his system” is far too vague to make any kind of judgement on Trayvon’s condition at the time of the attack. THC stays in the body for quite some time after the last noticeable effect wears off. He could have smoked up anytime in the three days (or more) prior to the attack and he would still test positive.
It is more evidence that he isn’t the choir boy the MSM wishes he was, but it isn’t damning evidence of his guilt.
THC IS FOUND IN THE BLOOD ONLY FOR TWO DAYS.
See http://www.passyourdrugtest.com/timetable.htm
Even if he was proved positive to have been stoned it wouldn't prove guilt of anything but being stoned. It does prove George Zimmerman was credible when he said Trayvon looked stoned.
I Googled a little bit, and came up with this link/story. About 1/4 of the way into the CNN story, THC and THC-COOH are mentioned. According to this story, were he caught driving in Nevada, he would be "impaired".
Another link, here, seems worthwhile reading regarding THC and THC-COOH levels, impairment, etc.
There may be more, studies and documents, to be found through web searches; but it's late and I'm tired. I'm sure others here will dig around for more info.
From a look at the lab report, the THC level was pretty low. (1.5ng/ml) and THC-COOH also low (7.3ng/ml)