Psychopath
Psychopathy (/saɪˈkɒpəθi/) (also known as, though sometimes distinguished from sociopathy /ˈsoʊsiəˌpæθi/) is traditionally defined as a personality disorder characterized by enduring antisocial behavior, diminished empathy and remorse, and disinhibited or bold behavior. It may also be defined as a continuous aspect of personality, representing scores on different personality dimensions found throughout the population in varying combinations. The definition of psychopathy has varied significantly throughout the history of the concept; different definitions continue to be used that are only partly overlapping and sometimes appear contradictory.[1]

A person with an antisocial personality disorder, manifested in aggressive, perverted, criminal, or amoral behavior without empathy or remorse. 

Psychopaths tend to lack normal human emotions such as guilt. They are also often highly intelligent and skilled at manipulating others. 

Also, psychopaths seem to appear normal. You would probably never guess there was something wrong with them.