There is a difference between dark and evil. They are not one and the same. To explore evil, you have to be prepared, because in exploring it, you explore yourself and the evil that exists within you.
Evil isn't "bad" or "malicious" or "vengeful". It's an intense malignancy. A unchanging state of pure adversity to anything good. It's a state so pure it is impossible for a conscious thing to truly achieve.
When I think of real evil, it is not physical, but psychological. It is never the action that is evil, but the intention. Many reasons can go behind the murder of a child, but there are only few evil reasons. Even Satan is not purely evil. Sometimes he is simply a creature whose pride was hurt and takes it out on humanity. Sometimes, he is an employee of God designated to work where few others want to. Rarely is he a being who descends to pure, unprovoked evil.
I think creating an evil character is hard because it isn't controllable. It has no allegiance or true motivations. Having those things means it has goals and by having those it is able to restrain itself from being evil continuously in order to exact that. Being evil, it would be impossible to have allies. Being evil, it would be a creature of pure impulse, moving in one single vector as chilling as any other impossible thing in the natural world.
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