Dark Eldar morality seems to be to be one where the only normative value judgements are those of pure self-interest - not even a speciesist theory, such as the Craftworld Eldar, but one closer to some sort of madman's interpretation of Machiavelli... While Regan and Singer would dismiss the Dark Kin as moral beings, they might be forced to admit that they are at least consistent on a level that the Craftworld Eldar, the Imperium, and our own society is not. We draw artifical distinctions of species, class, family, nation, and the like, and then say "THIS, on this side of the line, is valuable - THAT, over there, is not", no matter the relative merit of those individuals who fall on each side of the line (Regan and Singer being, really, atomists of ethics...). The Dark Eldar are also atomists, but consistent ones. The only line they draw is the one around themselves. They may even admit that someone else is better than them, for some ethical value. However, that then only makes them a risk or a target...
For information on the Dark Eldar, I suggest reading One More Hit, on the IC boards here, Dark Magenta, and of course the Codex. Xenology has a couple interesting bits - like that while the restrained and puritan Craftworld Eldar call Slaanesh "She Who Thirsts", the more emotive Dark Kin call her "The Thirsty Bitch".