We often say things like "the person who gunned down innocent bystanders at (name Any recent event) was mentally ill" because we're simply not comfortable or emotionally equipped to deal with the truth.
Sit these shooters down with some psychiatrist or clinical therapist, and more than likely, you won't get a diagnosis of a personality disorder, or PTSD, or schizoaffective. You likely won't even get the DSM's flavor of "psychopathy" or "sociopathy." You'll likely get nothing even remotely remarkable, except maybe what Stephen King calls a "howling nothingness" behind their eyes, a demonic slyness hidden deep beneath layer after layer of ideological justifications and defense mechanisms; a willful ignorance of any metric of kindness, decency or compassion.
Stop stigmatizing those who struggle with their mental health by conflating them with monsters. Some things are what they are. These people aren't mentally ill, they're evil.
