My Wife and I love this quote from Dr. G Simon:

Most disturbed characters don’t hear that little voice in their heads that urge most of us to do right or admonish most of us when we’re contemplating doing wrong. They don’t “push” themselves to take on responsibilities, and don’t “arrest” themselves when they want to do something they shouldn’t do. If they do hear that little voice, they can silence (or “compartmentalize”) it with great ease. But for many disturbed characters, that voice is quite weak in the first place. In the most severe disturbances of character, conscience is not simply weak, underdeveloped, or flawed, but absent altogether. Even the capacity to form a conscience is sometimes nonexistent. Dr. Robert Hare aptly named his book about the most severely disordered character, the psychopath, Without Conscience. It’s hard to imagine there are individuals with no conscience at all. That’s one of the main reasons such people are able to prey upon others. Very few can believe that the person they’ve been dealing with is as heartless or remorseless as they intuitively might suspect.