"Can we agree that all terrorists have some degree of mental illness?"
Er, no. I'd say the vast majority are completely sane and deploying violence against innocents to make a point, seize political power, gain territory or, increasingly these days, to praise Allah. All perfectly sane. That's the bottom line for understanding their evil. I take Hewitt's main point about our wanting to ascribe other motives, and second his refusal to do so. But I think it's important to remember that ethnic or, more often now, religious hatred is perfectly sane, if evil. The mass murder of Jews was carried about by completely coherent people on the whole. That also applies to an enraged Muslim gunman in Seattle. More biographical data here. A little eccentric, maybe. But sane.