Read The Sociopath Next Door by Martha Stout, ph.d., who states that 1 in 25 people fit this personality disorder. As with any mental illness, there are different degrees. One might kill, while another won't go that far but will rob you blind. I've known several people who fit this description- they don't have a conscience & are manipulative, compulsive liars.
Has anyone seen the show Dexter, about a serial killer who only kills people who deserve to be killed? In other words, a serial killer with a conscience. I can't watch that show b/c the concept is so stupid, it's an oxymoron, & I'm sure that it makes some people actually believe that serial killers have hearts.
However, I am glad that the actor who portrays the character in the series won best actor, b/c he has cancer.