I think it's a matter of potency, and of thresholds. Millions of people don't commit mass murder - most notice nothing. But amongst millions of people, there's a statistical percentage of those who will experience increased stress and instability.
They can be identified, manipulated, directed, and given a gun and a target by a network of handlers.
Its just a matter of coordination, and playing the odds.
Your points are well-taken. In statistics, these people who break under that chemical or radiological strain would be outliers, outside the standard deviation, but still would exist.