“The fact that somebody would go out and massacre a bunch of strangers, that’s not the act of a healthy mind, but that doesn’t mean they have a mental illness.”
As noted in the article, anger and resentment are not mental illnesses.
The right loves to go to this because it takes the conversation away from the availability of guns, while the left is open to it because it fits with the general principle that people aren't ultimately responsible for their own actions.
Link: https://www.washingtonpost.com/health/2019/08/05/is-mental-illness-causing-americas-mass-shootings-no-research-shows/