The scary thing is that when I watched documentaries of people becoming serial killers, they start to have a fixed, false belief of whom they are targeting and why. I asked my mom why when people kill three or more innocent people, something about their beliefs is delusional, and she explained to me that they have personality disorders that causes delusional thinking and that no one without those types of personality disorders would want to do that to innocent people.
Those personality disorders are Antisocial Personality Disorder and Narcissistic Personality Disorder, with symptom of delusions.
Those serial killers that got NGRI were diagnosed with personality disorders, with delusions as a symptom.
In the Antisocial personality disorder spectrum, Narcisist would be the less severe part of the spectrum (like ASD1 on Autism spectrum). Its very rare that narcissistics become serial killers.