I don't think UB is evil, but rather beyond morality. It's not exactly responsible for it's actions. It's the extension of a demon lord's will or something like that (the voice coming from that book that the guy found). UB is just a tool, and tools are not evil nor good. It's like saying a hammer is evil because you can hit someone in the head with it. If UB had Sedna as it's master, then it would probably be used to plant daisies and stop global warming...and I don't see the evil in that.
Sedna did not abandon the tribe from what I can remember.....she was outcast by that idiot chief or whatever he was. She left because she did not wish any more conflict.
Qween of thorns was born different. Because of this she could not find her place. No one of her pixie/fairy friends liked her. So when she found out that she could control other living things she took refuge in that. In other words she did not necessarily choose evil, but rather was forced into it so she could not be pissed on by her "friends".
Rook is the same as UB......not a sentient being that thinks on it's own, but rather a tool. Rook might be good now because he protected those people. The soul of that guy who died is controlling rook. If it would be replaced by a demon or something, we would see a tottaly different side of our friendly neighbourhood walking citadel.
Erebus I feel sorry for...poor guy . He is not evil, he just is classified that way by the people who are afraid and too ignorant to understand him (I'm refering to the people in the story, not you guys )
Torch Bearer.....well.... we all have a part of us that has dark thoughts and a part that has good thougts. His good part was killed in that fire, leaving only his evil one. So he is not really insane, just missing a half of himself. Evil he is, but not by choice, he is only a meat shell of his former self.
Oak is not an outcast, he was banished for not wanting to slaughter innocent women and children. He self sacrificed himself to protect them. Since he was banished he spends all of his time defending the people that were banished with him.
Dunno what to say about Regulus, can't remember his story that well.
Anyway, my point is that they were all good(except rook and ub) in the beggining (as I believe all humans are), but then they somehow got screwed by somebody. Some of them (sedna, oak) could still afford to be on the good side, other's had no choice (erebus, Qot).
So I think it's more like a moral story. You all start the same, but from there, it's anyones guess on how you will turn out, depending on what happens to you.