Clean your light bulb, bub. Even if every murderer smoked pot, that doesn’t prove every (or even most) pot smokers are murderers... what kind of craziness is this, maybe it rivals that of smoking pot.
The argument is that tampering with your brain chemistry is more likely to set off people in our general population who aren't wired all that well in the first place. In other words, it brings out psychoses that might not otherwise manifest.
That it worsens psychoses that do.