It actually gets even more metal - the reason for this is that before the Dark Eldar were a separate faction, the original united Eldar sunk so deep into hedonism and drug-fueled giga-orgies that their sheer drive towards ecstasy spawned a new Chaos God of Pleasure, who then proceeded to claim all their souls as food. So one particular group of Eldar decided to delay having their own souls eaten by instead making sure others suffer and are consumed in their place, hence all the extreme torture. Their society is also completely dysfunctional on account of everyone being a sadistic mass murderer, with politics consisting pretty much entirely of assassination attempts and other schemes and intrigues.