He's mad because his uncle used him as a puppet, and also because he didn't trust him with the truth of his plan.
The uncle first asks Taeui to join UNHRDO because he needs help get his superior (gentil) a promotion.
There seems to be two people fighting for this promotion, Mao and Gentil, with different supporters.
Before leaving Uncle asks Taeui to do work for mckeen, someone on Gentils side. This involves leaking documents of Jaeuis weapon designs, and Ilay catches them essentially setting a trap for mckeen and gentil.
Not only was uncle lying to taeui about whose side he was on within UNHRDO, he used jaeuis designs as bait and used taeui as a pawn.
Thank you so much for the explanation
Can someone explain what happened during ch64-70? I don’t quite understand why taeui was mad at his uncle..