If I was going to write extruded fantasy product, I'd have to write it from the point of view of the young lad growing up with poor but honest folks somewhere in middle earth who discovers that he's destined to grow up to be the Dark Lord, overthrow the established order, and start a revolution.
- Charles Stross
As I've previously noted, and Charlie goes on to explain in his post, the problem is not Dark lords, it's having any lords.
At the time Charlie's agent said this was going to alienate his readers. However 10 years have passed since then, and fantasy is dark and gritty and the new weird is being overtaken by the new swords and sorcery. Some authors have begun approaching this idea. The Steel Remains by Richard Morgan seems to be heading this way at the end (and it's publication date is around now, I type this so maybe we'll find out! Book Tralier here). K J Parker's novels are more tragedies, but from a certain point of view her protagonists who are so obsessed with their goals that they will tear down the world could be Dark Lords.
A really interesting one is in Well of Darkness, the first in the Sovereign Stone Trilogy by Tracey Hickman and Margaret Weis. It has interesting twists on regular fantasy tropes (Japanese Elves, Horse Nomad Dwarves, Seagoing Orcs) and a clear and smart turning to the dark side of the protagonist. It all turns crap in the second novel, and I never finished it. Something similar happened to this reviewer.
A Dark Lord is a fantasy supervillain. Supervillains tend to have the problem of motivation. Here though, that's easy. Nobles are bad. Even good ones. Magicians keep secrets, secrets that could benefit everyone. Elves tell us they're better than us, and when we ask for help, they tell us we can never be good enough. The king fights his war against the orcs, and we do most of the dying, but when we go home, he goes to a palace to hear songs of his deeds, and we go home to find we've missed the harvest, and taxes have been raised to pay for the war, and rebuilding the city. "If you do what we say, we will protect you" they say. And we do what they say, and we do all the work of protecting, and when we get home the village has been burnt by raiders and it turns out what we've been protecting is the nobles.
I think I'm on to something here.
 Probably not. But The New Swords and Sorcery is the subtitle of Swords and Dark Magic, an excellent anthology of stories from this subgenre.
 The real difference is that most of them aren't doing it because they think it's the right thing to do, as heroes do. Rather than gloss over the fact that saving the city will require the deaths of an entire nation doesn't matter, because they're just orcs, Parker's characters will acknowledge that these are real people and go ahead and kill them anyway. AND IT WON'T SAVE THE CITY.