An important part of novel writing is creating a realistic and believable enemy. In The Orchid, my editor praised the character of Clarence, who turned from a friend of Ava's into a man intent on destroying her. Cardboard cut-out villains are very easy to write, motivated by either money or just pure malice, they hide as a shadowed, malevolent presence on the side lines of the novel, appearing when necessary to make the heroine's life impossible. To create a memorable enemy however, you need to look a bit deeper.
Fiction is filled with famous villains: Dracula, Moriarty, Voldemort, and a personal choice, Heathcliffe. These men are not just evil however, but also deeply troubled. Dracula is shown at the end of the book as a man who was unable to stop the cruel manner he behaved in because he was a vampire. Heathcliffe was driven by a hard childhood and his love for Cathy to act as a monster to those around him. Given different life, these men could have been very different characters.
This is where the power of a good villain comes across. They are dangerous, often violent people, but they need a reason, a motivation, to act as they do. What drives your villain? What does he want and why?
Disorders like Narcissist and Psychopathic personalities are mental health issues. It's easy to label your character a 'psychopath,' without showing the true elements, or respect for those suffering these disorders. The person afflicted can't help the way they are, which brings in a element of pity, yet their behaviour can be very destructive to those around them. Their actions can be very secretive and subtle, so fit well in a psychological thriller or novel about verbal abusive/coercive control. The brilliant movie 'gas-lighting,' shows the lying and manipulative ways of a psychopath very well. They act as they do because they lack empathy and compassion for other people, but that doesn't automatically make them sadistic.
The 'Day of the Jackal,' is a novel about a remorseless killer, but there is one paragraph in the book where he is described with his face pressed against a shop window, staring at the display within. It was only a few lines in the book however it was enough to remind the reader that the man was human. A light touch is often all that is needed.