[She pauses a moment longer to clear away the remnants of a wall before she speaks again. It's a tricky subject: the worst of the anger is gone in the face of her removed bending, and its subsequent return, but she chooses her words carefully nonetheless. It's a feeling that's hard to forget.]

One of the bloodbenders who had his bending removed was a crime boss by the name of Yakone. Even with his bending gone, he taught his sons, Amon and Tarrlok, everything he knew.

I don't have all the details. Like it or not, you'd have to talk to Korra or Tarrlok to get them. But by the time Amon arrived in Republic City, he'd managed to perfect the art of removing someone's bending via bloodbending. He hid his status as a bender and gathered a following, claiming that bending was the source of all evil and that he'd been given his abilities by the spirits to nullify it once and for all.

He started with triad leaders and a few small-time bullies himself. Gathered popular support. Then he destroyed the pro-bending arena and went for the city's leaders--to say nothing of the Avatar herself.