A clear link exists between mental illness and many mass murders. Adam Lanza, the young man who massacred children and educators in Newtown, Conn., was known to have had some mental health problems.
But what of the Boston murderers? At this writing, why they engaged in wholesale slaughter was not known. Family members and others who knew them were at a loss to explain.
Something instilled a viciousness beyond most of our imaginations in them. One of the killers was videotaped placing a bomb beside the 8-year-old child who was one of three people killed in the blasts.
What contorts a human mind to the extent such evil is possible? Can such a conversion be spotted and treated?
It may be the killers, in their own minds, thought their act was justified for religious and/or political reasons. What we call brainwashing by cults - and that is what some terrorist organizations amount to - is not uncommon. It has been studied exhaustively. And by any reasonable view, susceptibility to it is a sort of mental derangement.
Of course, those who with cold calculation, drive others to terroristic acts - while doing all they can to stay out of danger themselves - are not mentally ill. They are evil.
But most acts of terrorism are not committed by leaders, but instead by followers. What is it about some people that makes them engage in brutality at the behest, either overt or perhaps only imagined, of others?
This is not a philosophical question. Again, anyone capable of doing what the Boston bombers did is mentallly ill by the yardstick nearly all of us employ.