Harvey Cleckley's The Mask of Sanity [claims] that there is a kind of evildoer called a psychopath, who does not seem in any way abnormal or different from other people but in fact suffers from "a grave psychiatric disorder," whose chief symptom is the very appearance of normality by which the horror of his condition is obscured. For behind "the mask of sanity" there is not a real human being but a mere simulacrum of one.
[But] the concept of the psychopath... evades the problem it purports to solve. To say that people who do bad things don't seem bad is to say something we all already know: no one flaunts bad behavior, everyone tries to hide it, every villain wears a mask of goodness.
The concept of the psychopath is, in fact, an admission of failure to solve the mystery of evil -- it is merely a restatement of the mystery.
from Janet Malcolm's The Journalist and the Murderer