In psychiatric terms, a difference exists between a narcissistic person and a narcissistic personality disorder. As with many psychiatric problems these symptoms exist on a spectrum. Many of us have some of the characteristics, or perhaps even a "trait disturbance," but they become a "disorder" when those traits begin to interfere significantly with loving, relating, and working.
A person can be supremely (and annoyingly) self-confident without it interfering in any large measure, but at some point, an invisible line can be crossed. Vanity can be a bit disturbing, but may be tolerable in some instances.
But for a narcissistic disorder, the world spins on that person's axis. Compassion and empathy don't exist except in their wish for OTHERS to be compassionate or empathetic toward them; it is never reciprocated. Then it disrupts their lives and the lives of those around them and for those for whom they are responsible.