The OP was an effort to defuse the barely-coherent rage that consumes me when people make the off-the-cuff assumption that "sickeningly evil" automatically equates to "crazy/mentally ill" and that somehow or other the law is expected to deal with it coherently, consistently, and fairly on that basis.
The man who committed that terrible crime may be mentally ill by a DSM standard, particularly as the APA in its endless quest to get treatment paid for by third-party payers has been 'medicalizing' every conceivable type of disruptive feeling/behavior relationship. He may (although I doubt it) fit one of the many legal definitions related to diminished mental status or capacity, or criminal insanity. But neither of those qualifications actually speaks to whether an actual brain disorder was the predominant basis for his act. Nor yet whether that relates to a diagnosed condition, nor yet whether such a disorder actually rendered him criminally insane by Wisconsin's standard. Nor yet whether qualifying under that standard will "get him off easy" in terms of... what? Punishment for what he did?
Being a person who did what he did is its own punishment. Society's thirst to equate punishment and vengeance with "justice" must not be further integrated into the law, which MUST function above such motivations if it is to advance civilization.
Reading the casual, shocked, uninformed, unthinking references to mental illness and insanity as a factor in that case was a major precipitant of the OP but not the entire underpinning.
pessimistically,
Bright