Nagira is a traitor. He betrays humanity every time he helps a Witch slip through Solomon's grasp, every time he shields a Seed from the necessity of constant surveillance. He betrays Amon, by daring to forgive what Amon can not.
Nagira lives betrayal every day, and smiles, unrepentant.
Amon, too, is a traitor. Amon betrays his duty, his convictions, the promises he has made to himself and others. He should report Nagira's activities, thinks about doing so (sometimes, not half so often as he thinks he should), and never does. He never does, and never will.
Nagira is a traitor, and he is the only family Amon has left.
In the end, family is worth more than all of Amon's rage.