Perhaps the coroner in question didn't notice that the effects of alcohol - which he actually admits the shooter was suffering from - frequently include mindless violence. In fact, that people's usual urge to get the hell away from strange drunks is for their own preservation.
So, he is positing someone for whom
a) Alcohol didn't bring on violence, despite violence occurring, and that
b) The violence wouldn't have occurred if the person had been both sober and not on drugs, and therefore
c) The violence must have occurred due to the cannabis.
Geeze, give me strength. What is the guy on? It doesn't look like this planet.
