Sociopath. It's a word that's tossed around a lot – usually as a synonym for "psychopath," but the two are not the same thing (although they are closely intertwined).
Both types of people have Anti-social Personality Disorder, which means they have little to no concern for others, and engage in dangerous behaviors including manipulation and exploitation.
The difference between a psychopath and a sociopath, however, is conscience.
The psychopath has none.
Here are 10 therapists on their frightening interactions with sociopaths.