Warning: This is going to be a very philosophical post, I think. Just so you have been warned.
But why am I thinking and writing about evil at the moment? I've read "The Mephisto Club" this weekend (and some of the week, too) and this novel deals with the concept of evil, too.
I have made my way through this area instinctively ever since my youth. 'Evil' is a concept I have studied in the past. You can't even attempt to write a crime story without thinking about it. And you can't read crime or horror stories without encountering that concept, either.
One of the concepts is the idea of 'inner' and 'outer' evil. It's a concept a lot of horror stories work with.
'Outer' evil is something that hits the hero of a tale without any committed sin. The fate of Jonathan Harker is a good example for this 'outer' evil. It's not a sin or a need or a guilt that has brought him to the castle of Count Dracula, where he almost perishes. He's simply there because his job demands it. Dracula and the three brides happen upon him, almost destroying him completely. He escapes with his life, but almost looses his sanity. Of course, that's not really fair - but then, life isn't fair either, is it? 'Outer' evil is like a meteor strike: it can hit everyone everywhere and without a warning. There's no real protection against it.
But to the moralists working with movie scripts, 'outer' evil is unsatisfactory. If someone has to die in a movie, the person should deserve it - especially if it happens to be in a horror movie. That is where the concept of 'inner' evil comes in. There is something hidden within the person to cause the evil. A good example for this is Dr. Frankenstein. He build a being out of decaying flesh, against the laws of nature and god. So, in the end, he has to die. He has to be punished for creating such a monstrosity. And in the novel "Dracula" there's also a being personifying 'inner' evil: Renfield. Renfield the madman, the man who eats animals alive, because it gives him life - or so he believes. Renfield who invites the vampire in, delivering the complete household to the Count. Renfield who dies bloodily trying to take back this invitation in the end.
'Inner' and 'outer' evil work great when it comes to horror stories, no matter whether they are on paper or on the screen. Sometimes both areas are combined. Think of "Jaws": It's nobody's fault the shark turns up, but it's the mayor's fault the beaches are not closed until the danger is removed - thus a lot more people die. And people who make such decisions almost always die as well.
But when it comes to thrillers and crime stories, the rather simple concepts of 'outer' and 'inner' evil do no longer work. Humans are too complex to be explained that easily - and unlike in a horror story, the 'monster' in a crime story or thriller is human. Not a vampire, not a being build from decaying body parts, not even a benign doctor taking a little elixir so he can party down without being recognized (the basic story of "Dr. Jekyll and Mr. Hyde"). This Dr. Jekyll doesn't need an elixir to become Mr. Hyde. He needs a closet ... or a phone booth ... or simply people looking away for a second.
I'm not going to discuss what makes humans Humans. That's a bit too philosophical, even for me (at least at this age, ask me again in thirty years or so). But fact is even a mass murderer or a serial killer is human. He or she (more likely he, though) may not be Human, but human in the biological and genetic way they are.
We are able to murder other humans, it's what sets us apart from most animals (only some primates, mostly chimpanzees, are also capable of willingly killing someone). A lion can kill a gazelle, of course, but it won't be murder. It's survival and both the predator and the prey know their place. And that might be where 'evil' starts, once killing isn't for survival any longer.
So, what other faces does evil have?
It can look ugly and beautiful, it can come from the highest and the lowliest places around. A serial killer might live in the slums or in a villa, it doesn't make a difference when it comes to the evil in his work - although it may make a difference to how he does it.
Cain slew Abel, the bible tells us. (And only very few people can name the third son that must have existed, as Cain is not the father of all mankind - it's Seth.) This is the first murder, the first real evil done outside paradise.
What happened to Cain afterwards? He was not killed himself, but exiled and marked. And this is where our idea about murderers being recognizable somehow comes from. Cain was marked, so that everyone could recognize him as the killer of his brother - and they were not allowed to kill him. He was to be left alive and could start his own family. (Although details are sketchy as to where he found a woman to have children with - Adam and Eve being the only other humans on earth.)
The modern killers are not marked. There is no face of evil. Quite often they look harmless. They behave harmless. It's only the moment the mask of normalcy falls, the moment the weapon is raised and the victim's life is about to end, that the monster jumps out of its closet.
But then, why do people commit evil? Why are there mass murderers out there and serial killers? Why are there rapists? Why do some people revel in destroying other people, physically or mentally?
That is the real problem in the whole discussion about evil, the real problem in writing such stories. What makes those people tick? Can you be raised to become a killer? That's what happens in "The Mephisto Club". Can a family's bloodline contain such evil and produce killers? That's what happens in "Body Double" (also by Tess Gerritsen).
In essence, the question might be: Is there a natural born killer or are killers made?
There is no answer to this question. Or rather, there are two: Killers are born and made. It depends on your idea of mankind which is true.
Evil can come from outside - most victims of the serial killers of past, present and future have not (or will not have) deserved their fate. Evil often comes from inside, one sin, one guilt growing until it destroys the being or makes it destroy others. Evil can be a seed already planted in a child, making it grow into a killer. And it can be instilled into a child, turning innocence into blood lust.
Evil has many faces - Satan, Lucifer, Seth (not the third son of Adam and Eve, but an Egyptian god of war) are just some of them. Some might count Loki among them (but the gods of the Norse mythology are neither completely good nor completely evil). Ahriman surely is one of them (and the Persian philosophy of Zarathustra is the first one centred around a good god and an evil one). And everywhere throughout mythology and religion there are many, many others, because humans always saw that the evil was among them, that murder and crime were to be found among them.
There is no such thing as the face of evil. And it has as many faces as there are humans around, because everyone of us could be evil.