Narcissism=/= Narcissistic Personality Disorder
The former is simply what you call someone with very high self esteem. Everyone is ultimately self-serving sure, but not everyone has high self esteem or sees themselves above others.
However, clinical NPD is much different. It's caused by emotional neglect of a child by one of the parents. This causes the child to create their own version of reality.
If Narcissism is a high self esteem person, then NPD is low self esteem person who lives in la la land. Imagine a person that sees themselves above others 24/7, never takes blame for anything, goes into a baby rage whenever someone impugns their name, along with being very shallow and severly overestimating their abilities. That's NPD. Trust me it's not someone you want to be around.
Being a Psychopath doesn't actually make you automatically Narcissistic. Psychopathy has certain subtypes. A psychopath with a big ego is also a narcissist, whoever not all psychopaths have egos and not all have NPD.