You'll know the dark ones by their behavior. For instance, a man who tortures a dog by, say, forcing him to endure being eaten alive by sand fleas is dark. A man like that wants to SEE what happens. He doesn't want to KNOW what will happen; we ALL know what will happen. The difference between him and us is that, rather than being horrified by the mere mention of such a thing, he wants to SEE such a thing, and the dog will serve to satisfy his desire to see such a thing.
We all know that a monster can be defined as a being who has a cold indifference to the suffering it brings to another. The behavior of the man described above is necessarily the result of a disconnect. The man has lost his connection to life. Along with that, he lost the capacity to identify with living things. Disconnected humans engage in disturbingly disconnected acts of ignorance because that's what they want to SEE.
