This sounds promising, but the problem with this approach is that it isn’t just mentally ill people who commit mass murder. People who are angry or desperate, or warped idealists who truly believe the world will be better without certain people in it, are the ones who kill. Are those emotions far from any of us? We’ve all felt angry, or desperate, or fed up with certain people — just not enough to overcome our desire to not be killers.
I would argue that those who do not stop at normal anger and instead act on it in an extremely destructive way to the point they want to kill others are in fact mentally ill to at least some degree. That level of hate and anger is not normal.