>Mental illness is described in terms of deviance from "normal" behavior and conformance to generally accepted morals. I think this is trivially proven from a lay read of the DSM.
Deviation from normal behavior and generally accepted morals is absolutely not a definition of mental illness. A bank robber isn't mentally ill by virtue of robbing a bank. I don't believe you've ever read the DSM in any of its editions.
>No conceivable context would make that chain of actions acceptable.
No one said they were acceptable.
>How could one look at this action and assume mental illness does not exist?
People do horrible, outrageous things without diagnosable mental illnesses all the time.
>Saying mental illness exists based on their behavior is not even close to the same thing as saying that they did not have culpability for their actions, and it's both unfair and disingenuous to imply I did.
The assumption that any horrible, irrational crime must be due to mental illness--obviously this diminishes culpability. Have you never heard of pleading insanity? If someone is genuinely out of their mind, they cannot be held responsible for their actions in the same way a normal person would be held responsible.