The Dark Avatar caused the Cataclysm
If another dark avatar was reincarnated after Unalaq it could explain why everyone blames "The Avatar" for the cataclysm. While we didn't see it in S2 of Korra, as Unalaq only knew how to water bend, theoretically nothing should prevent the dark avatar from learning and wielding all 4 elements if Ravaa and Vaatu are truly two sides of the same coin. To the average Joe, anybody bending all 4 elements is obviously the avatar. This confusion is probably how the hatred of the avatar started. Everyone conflated the dark avatar, who brought about the cataclysm, with the real one.
What we saw Vaatu do in S2 of Korra seems (to me) identical to how Ravaa and Wan started the avatar cycle. Vaatu was also never sealed away again in the spirit world. On top of that, with Pavi and Nisha being twins (and their cats being color inverts of one another lol) the picture becomes pretty clear.
To conjecture even more, if Korra was out of the picture on some Toph timing and was just chilling in the wild for 100 years (lord knows she needs a break), people would assume she's dead and thus never realize that there are 2 avatars at the same time. Similarly, if Korra's unplugged from society, she would never realize that there's a dark avatar until they start destroying the world.
I think this wraps up everything pretty neatly