The Dark Avatar as an ideal was never well represented, but, in The Legend of Korra I feel like the Avatar is viewed differently. In The Last Airbender, it's viewed as hope, salvation, inspiration, balance and the villains view the Avatar as a threat for those reasons (that and the physical bending threat). In LoK, everybody in the series, save for Tenzin (I would presume it's because he's old-fashioned) and the old characters (Toph, Katara, Iroh, and Zuko), seems to view the Avatar as a symbol of power. Even Korra during Book 1 and the beginning of Book 2.
Amon sees the Avatar as the ultimate bender, and the Equalists equate (hah) that to ultimate oppression. Zaheer sees the Avatar as a sign of balance as well, but also a sort of ultimate authority figure, (which he opposed) and even tells Korra her power is limitless. Kuvira sees the Avatar as an outdated notion and really nothing more than a weapon. Tarrlok also saw the Avatar as a tool and weapon. Unalaq especially saw the Avatar as a symbol of power, probably more than anyone else.
Unalaq, I always saw as a pure megalomaniac. Everything was a means to an end: He got his brother banished because he wanted the throne. At some point he joined the Red Lotus, a group of powerful benders with ties to a powerful group (The White Lotus) and learned about Vaatu and Raava and the spirits (he probably learned Spiritbending during that time as well). he then bands with Zaheer's group to kidnap Korra to open the portals (which last I checked, Zaheer wanted as well), but that doesn't work and he saves his own skin. He then took the Southern Tribe, not for unity, but because he wanted the portals open. He claimed it was for spiritual balance, but it was just to get to Vaatu, which is why he needed Korra to open the portals. Unalaq only mentored her so she could do just that.
Unalaq then, at some point, makes a deal with Vaatu, but why? Vaatu sees bending as his downfall and needs a way to combat it. Unalaq sees the Avatar as the ultimate power. If they merge into one, Unalaq gets access to Vaatu's power and Vaatu now has bending and a way to fight it. Use that power to get rid of the current Avatar, and now there is no means of opposition (until ultimate cosmic power and magic Jinora bullshit gave us a cool fight). In fact, if I recall correctly, didn't Unalaq tell Korra she was a garbage Avatar? He was jealous of her because of her power and it probably didn't help that she was his brother's kid, someone he already took power from once. Unalaq effectivey steals Tonraq's power twice, and then takes his daughter's. Vicious asshole, that one.
I always liked the idea of Unalaq manipulating Noatak to remove benders, so as to limit Vaatu's positiion. Then, after Noatak would steal Korra's bending, Unalaq would help her reconnect with Raava and open the portals. Instead, he settled on training Korra in her spiritual abilities. Different method, same result. Would have been a nice way to connect the books. Sadly, I'll never get that connection.
