NPD is dangerous because....

"Harm" or damage doesn't necessarily have to manifest as physical violence.  Emotional abuse is quite common and typical with narcissists.  You answered your own question:  absence of empathy is the basis of their "danger."  This means that the NPD will say and do whatever provides the response that they want, whether it is physical abuse, or emotional abuse.  Abuse is abuse - the damage of emotional abuse is not visible.  When someone leaves a bruise or broken bone, the victim can point at the injury and say, "This person did this to me."  There are no visible indications of emotional abuse, thus making it much more insidious. 

A child of a narcissist often experiences intense and overwhelming feelings of inadequacy and disapproval.  Nothing that the child does meets the unrealistic standards of the N.  This, and many other behaviors, causes the child to have serious self-esteem issues, as well as countless other emotional issues that are HARMFUL.  The DANGER is directly related to the damage that is willfully inflicted upon "source targets" by the NPD.

Additionally, human beings are predisposed to narcissism - it is a survival mechanism that each human being has.  However, the difference between "normal" narcissism and malignant narcissism is vast and complicated.

I recommend "The Sociopath Next Door" if you are experiencing concerns about a family member who is full-blown NPD.