If mushrooms made him flip that bad I suspect he may have had other serious problems that were going to eventually manifest themselves regardless of any drug use.
I wouldn't blame the drugs. The guy's clearly sick, and the mushroom tea is just incidental to the story. Mushrooms just make you happy, and maybe hallucinate if you take a lot. Or they occasionally make you freak out- but not to that extent. And you become scared rather than aggressive.
You see, that kind of comment makes me think that maybe the venom you're directing at this person (who is, admittedly, a heinous criminal) is just a smokescreen for your own dark, violent fantasies.