When he was convicted, he resisted. He overpowered the whole courtroom and almost escaped the city before Avatar Aang caught up with him. Aang took his bending away. [Which... probably is a bigger state secret for the most part. But after the story Korra had and then the one she learned later from Korra about his origins, there's that] He was taken to prison after that, but a few years later his gang helped him break out. [...]

Just so you know, everything I'm telling you after the trial I know secondhand. But apparently he disguised himself and started a new life up at the North Pole under a different identity. He found a wife, had children - two sons. And he secretly trained them both to be bloodbenders. One of them was Tarrlok and one of them grew up to become Amon.