Amon and Tarrlok are brothers. Amon is Tarrlok's younger brother, thought dead after an attack by a rival gang that killed him. In reality, he was put away by his father who couldn't bear to look at him after a firebender horribly scarred his face. Already bitter and jealous because he was born a non-bender, he devoted his life to finding a solution to the bending "impurity."

This is also why Tarrlok could not hurt him. He shared the same blood as him, and therefore was unable to fully command it.

Boom.