As the thesis of this blog states: psychiatry is politics.
I'd like to offer an idea for consideration.
The reason there's so much give and take about whether Cho was ill or not, and whether he was culpable or not, has to do with what psychiatry actually is: the pressure valve of society.
Our society does not have a good mechanism for dealing with poverty, frustration, and anger. I'm not judging it, I'm not a left wing nut, I'm simply stating a fact; ours is not a custodial society, and it does little to "take care of" (different than help) these people.
So it has psychiatry, it fosters psychiatry, and it creates a psychiatric model in which these SOCIAL ills can be contained.
The inner city mom who smokes daily marijuana to unwind, with three kids who are disruptive, chaotic in school, etc-- society has really nothing to offer her. But it can't let her fester, because eventually there will be a full scale revolution. So it funnels her and her kids and everyone else like her into psychiatry.
Whether she "actually" has "mental illness" or not is besides the point. Without the infrastructure of psychiatry, hers would be an exclusively social problem with no solution. But with the infrastructure of society, her problem is no longer a social problem, and no longer the purview of the government (or fellow man, etc)--it is a medical problem.
Consider that one of the fastest ways for this woman to get welfare-- and ultimately social security-- is for her to go through psychiatry.
So, too, the angry, the violent, the frustrated...
Hence, discussions about whether mental illness reduces culpability are red herrings. It's about reducing culpability, it's about reducing society's obligation to deal with it.
Society is basically saying this (I'll quote myself):
...if they're poor or unintelligent, we will never be able to alter their chaotic environment, increase their insight or improve their judgment. However, such massive societal failure can not be confronted head on; we must leave them with the illusion that behavior is not entirely under volitional control; that their circumstances are independent of their will; that their inability to progress, and our inability to help them isn't their (or our) fault; that all men are not created equal. Because without the buffer psychiatry offers, they will demand communism.