I've often had that thought. My ex was not a narcissistic sociopath (although he certainly has his problems), but I've met people who are, and they are far more frightening than the obviously mentally ill. I think eventually though they all get tripped up - the lack of empathy means that they can't really fathom how other people think or feel, and they can only fake it for so long.
I remember hearing a radio interview with Karla Homolka after she was released from prison in which she was very convincing as a pathetic victim of Paul Bernardo ... until the point when she said that she was terribly upset because he used their wedding china to prepare a meal for some poor girl he was abusing. Homolka was clearly a good learner and had figured out how to sound like she had compassion, but there came a point when it became evident she really had no understanding what remorse or empathy were.