Thrawn has an outburst of rage emotion when his underling claims that the Twi’lek family heirloom is trash and that it should be thrown out.
That scene seems rather odd at this point. The idea is to show Thrawn as menacing toward poorly trained Imperials incapable of doing their jobs, like Slavin.
Slavin is caught off guard and fears Thrawn will kill him. The red eyes are deadly. But Thrawn backs away and attempts to put Slavin at ease. It does not.
Thrawn studies the Lothal Rebels to destroy them — piece by piece. To do that, he encounters Hera Syndulla and Ezra Bridger.
Thrawn was specifically requested by Arinhda Pryce, and Tarkin allows it. Thrawn takes his mission seriously and does not judge how missions can advance his career. His objective is clear, protect the Chiss by serving the Empire.
Thrawn becoming a Grand Admiral has no bearing on his vanity. Thrawn is not vain. He calculates strategically for the best tactics to achieve his mission’s objective – protect the Chiss through the Empire in the galaxy from threats.
This scene from Star Wars Rebels ruins his ability to reach his mission’s objective. Incompetence is intolerable in a society that holds a government organization and concomitant military. And Thrawn seems to encounter more incompetence since he was ordered for this mission. This fact is disconcerting to Thrawn because while he can clean it up, will incompetence affect him like a contagion, or is he predisposed or tainted to be fail at his mission.
He feels the conspiratorial thoughts, but review the facts — Captain Slavin is derelict in his duty.
Organizations have utterly lost wars due to incompetence and dereliction of duties. The Emperor has no desire to lose what he has gained.
Thrawn KNOWS this fact. But the longer he is around humans the more his emotional state becomes like them through their contagion. His emotions lack “being Chiss”.
At the same time the Chiss had exiled him, even though he lies to himself about it. They have been duplicitous. The Aristocra must be cleansed in the current Ruling Families. New generations of Chiss are dying precipitously and it is not sustainable (headcanon). But Thrawn’s behavior and personality despite all his training and grooming, his heredity is still tainted.
(Though in real life heredity and evolution does not scientifically operate that way).
The Chiss Ascendancy egregiously practices eugenics and has lost genetic psychiatric bio behavioral neurologically diversity which results in a congenital defect-depression that could lead to a population bottleneck and result in extinction.