Zhou Tian believed he had been good to the Beastmen; they should not have betrayed him. Yet, all this was merely Zhou Tian's thinking. To those Beastmen, Zhou Tian's rule, while not brutal or inhumane, was certainly not satisfactory. It was precisely because of their dissatisfaction with Zhou Tian's governance that they chose to collaborate with the Deity and betray Zhou Tian.
Thus, having taken that step, the Beastmen, according to their philosophy, did not believe there was anything wrong with their actions. For this reason, when confronted by Zhou Tian's questioning, the Beastmen did not show any sign of guilt.
Zhou Tian felt betrayed by the Beastman leaders, but they felt justified in their actions, believing they had enough reasons to betray Zhou Tian. With different perspectives, it was inevitable both sides would feel the other was in the wrong.
Therefore, when Zhou Tian saw that the Beastmen had no intention of repenting, his mood was naturally not very good.