Take a look at this fascinating article, written by Columbine author Dave Cullen, on the  mentality of killers:

When [James Holmes] walked into court Monday morning, one thing was immediately obvious. Something was wrong with this guy. Which was weirder, the dazed expression he wore most of the 11 minutes of the hearing, or the sudden bursts of wild eyes, matching his ridiculous orange hair?

The obvious explanation, which many viewers and commentators embraced, was that he was out of his mind or, medically speaking, undergoing some sort of psychotic break. But a minority view pushed back, and hard: the hair, the eyes, the sensational getup for the attack were a little too cute: a cold-blooded killer, playing crazy.

You will never understand this man if you leap to either of these conclusions. Do not look for a unified theory of mass murder, a single coherent drive. It doesn’t exist. Examining all the mass murderers together yields a hopeless mass of contradictions.