I don’t have direct experience with comorbid clinical narcissism and SZ. My brother has bipolar disorder not schizophrenia. He does exhibit some narcissistic traits, but they tend to wax and wane with his illness.
To answer your question, SZ is a thought disorder, while narcissism is a personality disorder. The underlying cause for SZ is a brain disfunction usually occurring in adolescence and young adulthood, leading to hallucinations and aberrant thought patterns like delusions. The theory of the underlying cause for narcissism relates to vulnerable children with harmful parenting, and narcissism begins earlier in life as personality is developed. Aberrant behaviors and perceptions are related to preservation of their ego concept and they look to the world for validation. People with SZ tend to look within, and tend more to blame the world for lack of validation as a defense when they are decompensating.
A classic tell of a true narcissist is lack of empathy, while many people with SZ can be quite empathetic when stable. For example Dr. Amador’s brother with SZ died when struck by a car while helping a woman load groceries on a bus. The act of helping someone without some transactional gain is foreign to a narcissist, and likely wouldn’t happen.
This is not to say people with SZ can’t be self-centered and greedy when ill, it just comes from a different place and it’s more of a transient function. And narcissistic traits don’t necessarily accompany SZ. You can stabilize someone with SZ with medication and they can become less self-centered and ego driven. To my knowledge, less so or not so with narcissists.
My opinion is the narcissistic traits of people with SZ are less harmful to society as a whole as they tend not to be as socially adept and functional and largely harm loved ones and caregivers. While a clinical narcissist is more socially adept and can inflict emotional injury on larger groups of people.
Edit: corrected that it was Dr. Amador’s brother not son who died. I realized the mistake earlier, but see I missed fixing it.
Disclaimer: While I am a part-time caregiver of my brother and father with SMI, I was diagnosed with SZA over thirty years ago. I’m an advocate of appropriate drug treatments for SMI, but I feel they are incomplete treatments and additional CBT, supportive talk and psychosocial therapies are helpful where feasible. Any drug advice is from personal experience or research and not a substitute for qualified Psychiatric care.