As background, most people are yelling, "how the hell can you say this guy wasn't crazy? He was talking to imaginary friends, he thought he was an Ax, etc." As point of fact, these weren't delusions because he knew they weren't true, but that's a side point.
Psychiatric pathology exists on a spectrum. It's not binary "ill" or "not ill," and impairment in one realm doesn't explain impairment elsewhere. A diagnosis does not define all of your existence, or even all of your actions.
I should not, however, have said he wasn't mentally ill. What I should have said was he was not insane: he knew what he was doing, he knew what he was doing was wrong, and he had the ability to control himself. So he is entirely to blame, i.e., the mental illness, even if substantial, is incidental.
You might say, ok, he's not insane, but only someone with a mental illness would do this. It doesn't lessen his culpability, however.
Well, actually, it does: you can't execute the mentally ill, for example. Forget about your personal stance on the death penalty. Fact is, mental illness is rapidly becoming an exclusion to a sentence that everyone else is subject to. I know, it seems so righteous to say the mentally ill shouldn't be executed. Ok, here: it would mean you can be sentenced to death, but he can't. Does that make any sense, moral or legal?
Clearly, maladjusted and sexually frustrated college kids don't often go on rampages, so there was something in him that moved him to this. Perhaps that was the mental illness. But add up the body counts in the past twenty years. What's in common in mass murderers isn't mental illness, but frustration, impotence (metaphorical) and anger. Or are all those suicide bombers in Israel bipolar?
You'll say, "but he wasn't a suicide bomber." His mental framework had much more in common with a suicide bomber than with John Wayne Gacy.
But let's put this aside and ask a different question, about us, not him: why do so many people want him to be mentally ill? Because its an explanation that doesn't implicate society, or themselves. It means the world can be divided into "us" and "them," which is always fun. It's the easy scapegoat that seems to be so obvious as to be unassailable.
And if it is mental illness, what do we intend on doing about it? My bias implies harsher sentences, societal changes, etc-- we can debate that later. But if it is all mental illness, then what? Do we lock up the "mentally ill" like we do pedophiles and terror suspects, before they even commit a crime, just on suspicion? And who decides who is suspicious? Psychiatrists? Do you trust every psychiatrist to be good at this? Or should it be the government?
Would you have been happy-- I mean this in all seriousness-- if George Bush had Cho arrested last year for being a terror suspect? Which part of that bothers you? It would have been legitimate, because he was dangerous. So is it that he was arrested before he committed a crime, or that George Bush did it? See? This is what you'll have to contend with with these policies.
Oh yeah. Treatment. You want to make "treatment" mandatory? Great. Tell me exactly who should decide who needs treatment, and for how long, and what kind. And tell me how this treatment is going to work-- what is the specific end point?-- and for how long, and tell me what we should do when the treatment doesn't work.
You can't just make this stuff up as you go along, enacting policies which are politically expedient but destroy the society. Ask Vladimir Putin. Oh, wait, bad example.