I don't feel like reading through this entire thread...
Has no one mentioned the possibility that Tarlokk is Amons brother, and that their father is Yakone from the flashbacks? Yakone could have seriously hurt Toph or Sokka or both, and Zuko could have stepped in and killed Yakone, being the "Firebender that took Amon's father away" ...
Tarlokks face when Korra compares him to Amon just screams brother to me...
PS: I don't consider The Promise actual canon. And I never will, even if the creators of ATLA ever state that they do...That book ruins the show for me if its real. And that's saying something, since I consider ATLA one of the greatest stories ever told.