I literally found the best theory for this, ZOMG!
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As of this morning’s episode, I think I’ve been able to piece together what happened 42 years prior to the events of The Legend of Korra.
A man named Yakone is on trial for his crimes - crimes that must be serious indeed based on the looks he’s getting from Sokka and Aang. He sits in front of the Council (including the Southern Water Tribe representative, Sokka) and waits for their judgement, Avatar Aang watching the proceedings carefully from the audience. The police are there as well, providing security, because this is a dangerous man.
Yakone stands up to give testimony on his crimes.
And that’s when it all goes to philoprogenitiveness.
Everyone in the room save Yakone - even his own lawyer - are bloodbent into submission during the middle of the day. The important thing to note here is that Yakone is not the one doing the bending, as his hands are cuffed and he doesn’t appear to be using any sort of bending form, even beyond the fact that bloodbending dozens of people at once is a feat likely well beyond a single person.
Yakone is the head of the Red Monsoons, a bloodbending Triad, and they’re coming to his rescue.The Red Monsoons were one of the first Triads formed in Republic City, if not the first, after their founder Yakone rediscovered the art of bloodbending. Every full moon they terrorized the citizens of the city, extorting money, committing murders, and threatening the stability and peace Aang had created. It took a long time, but with help from his friends, Aang finally managed to track down and capture Yakone so he could face justice.
Now they’re putting him on trial at Aang’s insistence, during the daytime so they’ll be safe from any Triad attacks.
Or so they think.
Yakone has discovered a way to bloodbend without the assistance of the full moon. His gang comes to his rescue during the trial, when almost all of Republic City’s most powerful figures are in one room, and make it so Yakone can flee.
But it doesn’t work. Why?
Because Katara arrives and bloodbends the philoprogenitiveness out of Yakone himself, after investigating how they learned to do it and finding out that there’s a way to do it anytime. That’s why we haven’t seen her in any of the flashbacks yet.
The Triad relaxes their hold on the people in the room out of shock and fear - could one of them accidentally be subjugating their leader? - and that’s all the opportunity Aang needs to break free and go Avatar State on Yakone, their battle taking them all the way to the street.
But just like in his fight against Ozai, Aang overpowers the State to keep himself from slaying Yakone, and instead removes his bending abilities and banishes him from the city.
Yakone leaves. And five years later, Yakone has a son.
Yakone taught his son the secrets of bloodbending, even if he could no longer perform them himself, and when he died, the young and clever Tarrlok traveled back to Republic City and began making his way up in the world. This is what Tarrlok means when he tells Korra that there’s a lot she doesn’t know about him.
In addition to the physical similarities, there’s hints in Tarrlok’s own words that he is Yakone’s son. Why would he choose events five years before his birth to justify the creation of his task force? For someone who’s only 37, he certainly seems to know a lot about what happened 42 years ago.
Tarrlok isn’t interested in getting revenge on anyone for his father - or at least, not yet. All he wants is power. Korra was a valuable ally for him, and so he was content to work with her for a time. But it was after she compared him to Amon that he snapped and attacked her. Why?
Because Yakone pumped his son full of bitter stories about how terrible it is to lose your bending. Tarrlok is going to such extremes in his efforts to take down Amon because the removal of bending abilities is his berserk button, so to speak, and now a man has appeared in his city with the power to do the same?! Tarrlok won’t stand for that. He’s putting all his effort into making sure Amon goes down, no matter what he has to do in the process or how unlikely it is that he’s fighting actual Equalists.
And now the Avatar - the direct reincarnation of the one who took his father’s bending - has the audacity to say he’s the SAME as Amon! Tarrlok isn’t going to stand for that. He no longer needs Korra as an ally, she’s begun to cause him problems, and now she’s ticked him off. It’s time for Tarrlok to remove her from the picture.
So he takes her.
And he locks her up in the place in the mountains where his father fled after his banishment, and where Tarrlok was born.
You know. A place right “
Out of the Past”.
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