I've heard and read many different (professional) opinions on just what psychopathology is, some of which hold that a physically identifiable cause of aberrant behavior, such as a brain tumor, must be present to qualify. But I believe capability and motivation are adequate indicators of whether someone can be considered psychologically normal or not.
Timothy McVeigh, for example, was motivated by the BATF's and FBI/HRT's actions at Waco to spend months constructing a massive bomb and detonating it without regard for innocent casualties. While his motivation is understandable, the capability of carrying out such an elaborate act of vengeance with utter disregard for innocent victims is sufficiently abnormal to qualify as psychopathology. In other words, anyone who is capable of doing something that bizarre is a menace to society by virtue of his psychological condition. Is there need for a more technical definition of what insanity is?
Is anyone here willing to say this Holmes fellow isn't crazy? Why should knowing the cause of his pathology be necessary for it to qualify as legal insanity?
Unfortunately, while that might work for a CLINICAL finding of insanity, it's not the legal definition of insanity. Legally, you cannot be said to have had criminal intent, and therefore held responsible, if you are unable to understand the nature, quality, and consequences of your act. If, for example, you are so incapable of differentiating reality from fantasy that you are mortally convinced your next-door neighbor is an evil space alien, intent on invading the planet and stealing your body, so you kill him to protect yourself, you're legally insane and on your way to a mental hospital.
On the other hand, if you know perfectly well that what you're planning to do is kill people, that it's illegal and viewed as morally wrong, and that you're likely to go to prison if you're caught, but you decide to do it because you think you're a supervillain and so killing people is a cool thing to do and part of your job description, you may be nuts and not well-attached to reality, but you may still be sane enough for legal purposes.
Also, it's possible that, like Jared Loughner, Holmes may now be too unstable to be able to contribute to his own defense. Whatever his condition was at the time of the killings, he might now be too deteriorated to comprehend what's happening to him, and thus be unable to stand trial.