In fact 50% of the people killed by police are "mentally ill".
Also consider that having severe depression, autism, ADD, etc. are considered mental illnesses.
Alex's Nieto's "mental illness", even if existent, is not an excuse for his death.
I am going to gently push back on you, though, because you're making it sound as if Nieto had a completely unrelated historical diagnosis that is being used here as a smokescreen. Here are some details salient, I personally think, to this case:
* Nieto had been prescribed two antipsychotics. He had not been taking them at the time the police confronted him.
* Just three weeks prior to this shooting, Nieto had tased the ex-husband of a friend while they were putting their kid in a car. The victim removed the taser darts, and Nieto tased him again. The victim managed to get into the car and drive away as Nieto screamed and threatened him.
* The second of Nieto's two antipsychotics was prescribed two years prior, when he was placed on psychiatric lockdown after trying to set his parents house on fire.
There is mental illness of the kind that many of us suffer and manage, with varying kinds of assistance. And then there is uncontrolled, ongoing, and violent psychosis.
Nobody is arguing that Nieto's mental illness is a good reason for him to have been killed. But the notion that the police, dispatched to confront a subject reported in a 911 call to have been carrying a pistol, faced someone who did indeed refuse to show their hands and then did indeed draw a weapon is corroborated by Nieto's history.