It’s also not ableist because “mentally ill people” is not even close to a homogeneous group of people. There are thousands of different mental illnesses and more than 20 % of people in the USA are mentally ill.
No one is saying these people are all murderers.
Please stop trying to push this weird agenda of “some people are just born evil” or made evil or whatever it is you are trying to say. It’s a step forward to recognise how society can prevent criminal behaviour, for example by treating mental illness better. “Evil” does not exist. There is always a reason of why someone behaves the way they do.