Most crimes are motivated by unlovely impulses that are at least comprehensible: the desire for money, sex, respect, revenge. We don't do these things...But we can understand why people want to--we know what someone is after when they hold up a liquor store, or even kills their spouse for the insurance money. Understanding is not sanction: these crimes still have the power to anger and horrify. But they're comprehensible, and that comprehensibility is surprisingly comforting.
The alternative is Newtown. When one tries to picture the mind that plans it, one quickly comes to a dead end....Trying to climb this mountain of wickedness is like trying to climb a glass wall with your bare hands. What happened there is pure evil, and evil, unlike common badness, gives an ordinary mind no foothold.
Since we can't understand it, we can't change it. And since we can't change it, our best hope is to box it in.