I wouldn't put the theory aside so quickly. Combined with a considerable amount of alcohol, it cannot be ruled out that fly agarics have an aggression-increasing effect on people who are already very aggressive. That these mushrooms are highly toxic is a fairy tale created to deter people from using them. A girl once led me to a place in the Bavarian forest where these things grow, and we ate them right away. There were no physical problems afterwards.
The ingestion of hallucinogenic mushrooms by Siberian tribes of the Kamchatka peninsula and by Indians of the Mexican highlands has been carried out in ritual and orgy for centuries. Ødman and Schübeler have advanced the hypothesis that the furious rage of the Berserks in the heyday of Viking culture a thousand years ago was brought about by the same agency, specifically the Amanita muscaria mushroom. A few years ago it was found that these fungi contain bufotenine, or n-n-dimethyl serotonin, a substance which is under scrutiny at this time for its possible neurochemical role in the causation of schizophrenia. Recent observations on the intravenous injection of bufotenine in man disclose that it is an hallucinogen, and that its psychophysiological effects bear a resemblance to the Berserksgang of the Norsemen in the time of the Sagas. These observations appear to offer support to the Ødman-Schübeler contention that the famed fury of the Berserks was what we would call a model psychosis today.
Uh, there certainly are toxic substances in fly agarics...but probably not so much as to have the reputation it has of being a deadly mushroom. It also has hallucinogenic substances. And as far as I know, it's a completely different mushroom they use in Mexico (and peyote is the popular one anyways).