Okay, let’s say he has an actual mental illness (other than Narcissistic Personality Disorder, which seems pretty likely). Mental Illness does not exist in a vacuum. There’s a reason that people reported hearing voices in the 1500s heard God and saints but people in the 1960s heard aliens and the CIA. If this guy hadn’t been raised in a culture that taught him he was owed a woman and sex, this is not how things would have gone down.
Also, again, even if he has an actual mental illness, he was mentally competent – he made manifestos and plans, and IANAL but I’m pretty sure he’d be ruled competent to stand trial were he alive.
I’m not letting the cops off the hook on this though; they were told about this guy and did nothing.