I believe the mental illness part: flat affect in every picture I have seen of him.
Which doesn’t preclude him being a vicious hateful SOB.
The “mental illness” part is to remove all responsibility from the perp and ignore the likelihood that the next perp will be from the same group.
Did they claim the axe-wielding guy who attacked the NYC cops was mentally ill as well?
In both cases the attacks were premeditated and from the standpoint of the attackers, completely reasonable - they were attacking those standing in the way of their lives of crime, and were giving up nothing if they died in a hail of bullets.