I believe the one behind the Darkness is an actual being, but I don't think it's currently possessing Alaric, it just implanted it's Darkness into Alaric via Gilbert ring, and the power of the Darkness was able to bring out the darkest part of Alaric. As to exactly what it is, I'm going for Kitsune. Since Alaric has even tried to kill himself, it shows he's been driven mad by the Darkness, and the other Alaric has no memory of these events, which both sound similar to Kitsune possession. Perhaps the ring is similar to the Malach, the more Alaric dies the more the Darkness of the ring will consume him and make him entirely under the Kitsune's will.
I understand Esther's reasons for wanting to wipe out all vampires. I don't support her as in wanting all vampires dead, because then we'd have no show. I just don't think she's an evil heartless mother that deserves to burn in hell. She just has a flawed mindset believing that all vampires are abominations to nature and in able to restore balance to nature they must all die. In able for one to truly bring balance to nature one must inveiw a larger aspect of both sides and not just the dogmatic view of the witches, and I think that's what Esther will learn as time goes on, that not all vampires are the murderous abominations she's making them out to be.
I like Esther because she's very mysterious and has proven herself to be a great antagonist so far, I mean just look at the way she's pissed off some of this fanbase. I can also see a lot of potential for her character, especially if she turns out to be a Kitsune like I think she is. I also believe it's possible that she may feel like she has no choice because there may be a more powerful force influencing her.