Will we ever get any Dark/Evil Demacians? IMO, Lucian and Vayne don't quite count.
Lucian might be a "Knight in Sour Armor", but he's still a Purifier and definitely one of the good guys. He's just always emo because of what Thresh did to Senna.
As for Vayne, she doesn't go around killing just ANYONE, but only necromancers, dark wizards, and dark monsters. In some ways, she's like a Viz-Jaq'taar (a mage slayer assassin) from the Diablo series, and like Batman or Huntress in more obvious ways. But that doesn't make her EVIL; she's more an anti-hero.
But so far Demacia has no villainous characters in its roster. Before you say that it's laws would make such a thing impossible, keep this in mind: in a medieval society like Demacia, the laws would benefit whoever holds the best cards.
To give an analogy, in Tales of Vesperia, Magistrate Ragou is guilty of committing some absolutely atrocious acts (such as feeding villagers who can't pay taxes to monsters for his entertainment), but due to his political influence the kingdom's laws can't touch him. He meets his end when protagonist Yuri Lowell murders him in cold blood (an act that makes you love Yuri even more because he doesn't give a damn about the consequences; he just wanted that scumbag Ragou to get what was coming to him).
Now what about having a Demacian noble who is somewhat similar to Ragou? Namely, a Demacian noble who has a LOT of political influence such that the kingdom's laws can't really touch him. I think that'd be an awesome addition to the story, though whether it'd be worthy of another Champion is up to debate.
EDIT: Oops, misplaced section.
before he redeemed himself, was sort of the villain to them.