In order for a human being to pull the trigger on 20 kids + 6 adults, one of whom being the guy's own mother, something really wrong has to happen.
Ever read http://www.amazon.com/Killing-Psycho...dp/0316040932?
It's fascinating. The book discusses how much mental conditioning is required to get professional soldiers to fire on other professional soldiers. People have an extreme natural aversion to violence and killing. To circumvent that, requires either a deliberate process to wear down the aversion, or an emotional and/or chemical (same thing it's all filtered through meat) imbalance, triggered by any number of stimuli.