In Oath Breaker Thiazzi killed Bale, Toraks kinsman, and Torak swore an oath on his three souls (clan soul, world soul and name soul) and on his mothers medicine horn and his fathers knife that he would avenge Bale, and kill Thiazzi.
Thiazzi is a sadistic person. He enjoyed the pain Renn felt when he snapped her bow. He also took great pleasure on slowly killing Renn with smoke. He stole children from the clans of the deep forest and even made them fight each other. This is clear signs that he also lacks empathy.
Thiazzi is a very self-confident person. He doesn't think he needs the power to spirit walk since he believs he is powerful enough anyway. He have shown to be intelligent and manipulative, being able to lead the deep forest clans to want to go to war, and played as both the Forest horse and Auroch mage.
Due to all these traits, Thiazzi might suffer from some kind of personality disorder, like narcissism and pyschopathy. He shares many common traits such as lack of empathy, manipulativity, grandiose thoughts of himself and fearlessness, when he dressed as the forest horse mage gave back a staff to the auroch and they pointed all thier spears on him. However, he didn't see himself as the most powerful of all people sínce just like the other Soul-eaters, feared Eostra. When Renn asked him if the had told "her" about the Fire opal, he hesitated.