Admittedly, I've stayed out of this discussion because I don't know a lot of the facts. But there are a couple things I have to point out in this discussion.
THC only stays in your system a few days. It's been shown in urinalyses that a person can smoke marijuana over the weekend, and then it not show positive in a urinalysis on Monday. I've seen it myself.
Now, the more often a person smokes, the longer it takes to get rid of THC in the blood. It's been reported that a regular user can have appositive test up to 45 days from usage, while a few extreme cases have even been detected up to 90 days.
Basically, I said all that to say this: if THC was in his system, then either he recently used it or was a habitual user.
However, marijuana does not make you aggressive. It relaxes you and is a downer. Makes you hungry and paranoid. My guess, based off experience (no I've never smoked it, but have seen people who have) is that if he were under the influence of marijuana at the time, he'd have been less inclined to confront anyone, and would most likely not even have noticed he was being followed.