As I read through the Ascendency trilogy, it is becoming more and more clear that the Chiss Ascendency is as hateful as the Empire. And it's odd how few people call that out. I think it is because Zahn does a fantastic job at hiding it through Chiss POV, but even then, the Chiss are still incredibly xenophobic and controlling. Yes, this includes Thrawn, he isn't the saint that so many people like to paint him as and frankly could be argued as worse.
I keep thinking about Ar'alani admitting she never saw non-Chiss as people. She is brilliant and kind, but only to other Chiss. We view her in a purely positive light because the POVs in these books are primarily Chiss, who agree with her. Of course her mindset is normal amongst Chiss, of course it isn't questioned, of course Ar'alani herself never questions it despite her experience off-world. It takes a direct and pretty personal interaction for her to think twice, and even then it is difficult for her to accept the humanity of a non-Chiss. They are lesser in her eyes. They are lesser in the eyes of most, if not all, of the Chiss.
It is fascinating, it really is. It's an interesting look into a xenophobic society without the initial hate from the reader. Because xenophobia is born out of misunderstanding and perpetuated systems of ignorance. If a similar situation was told but through the eyes of Imperial officers, fewer people would be willing to see the nuances. Because Empire=Bad and anyone associating with it is also Bad, right?
But, propaganda and cycles of ignorance are also to blame. Not every Imperial Officer was born hating aliens. Hell, even TARKIN started out incredibly sympathetic to alien species according to the canon novel by James Luceno. But his family taught him otherwise, just as the Chiss Ascendency teaches its own children see other species as lesser.
This mentality from the Ascendency is also seen in Thrawn: Treason with how Eli Vanto is treated simply for being human. The majority of officers hate his existence, insist he must prove himself (despite being at a lower rank than he was at when with the Empire), and are distrustful of him. Very similar to how Ronan treats Thrawn in the same novel.
This isn't, like, a call to love Chiss characters any less, but it's a bit odd to imply that Thrawn, or any other Chiss, would be against the Empire for the same reasons the Rebellion is. The Ascendency doesn't like the Empire because it could encroach on their rule, their space- not because it's xenophobic and oppresses too many people to count. The two systems are remarkably similar, which may be part of why Thrawn was inclined to help the Empire. It is familiar, and a system Thrawn himself has never opposed, even without taking Legends into account.
(SIDE NOTE: PLEASE DO NOT BRING UP SPOILERS FOR GREATER GOOD OR LESSER EVIL ON THIS POST. I AM STILL READING THOSE BOOKS AND WOULD LIKE TO ENJOY THEM SPOILER-FREE)