I think the differences between a narcissist and antisocial pd are usually apparent. The psycopath is likely by adolescence to be displaying strong antisocial traits, and to have been in trouble a lot at school or in the community, and to display increasing criminal / violent behaviours culminating in a court appearance or incarceration; they have lack of shame or remorse and continue their victimisation of others in a prison or therapeutic setting. They are sadistic and believe to show pain is weakness which must be exploited.
the narcissist wears their pain and it is visible- in fact its where many of us get sucked in I guess, first thinking they are incredibly insightful and sensitive. But rather than a genuine connection we were being manipulated by someone who can turn this on and off at will. The narcissist has a n emotional range but it is shallow and inconsistent, sentimental.
The ns I have known have suffered deep shame or remorse- the pd aspect means they don't acknowledge or learn from it or reach out to others to put things right though.
That is what makes nvictims feel they have met a psycopath, it is hard to believe the interaction which seemed so meaningful to you means nothing to them.