With Thrawn, I think the thing people can get caught up on is that there are those who deem that such a thing as genocide is beneath him, goes against his moral compass, and those who think it's in character for him to be okay with it. And then we have that subsection of people within that latter group who clash on the viewpoint on whether it's something Thrawn does for fun/the power trip, because he can, or if it's something he'd only resort to if he deemed it necessary to accomplish his goals.
I myself have shifted with how I feel about it and find myself in the party of him not being opposed to it in concept, but he doesn't believe it to be the absolute solution unless he deems it so after a deep, critical analysis of the situation. My understanding of Thrawn is that he wouldn't be opposed to the destruction of Alderaan because his moral compass prevents him from seeing that massive loss of life as justified in any context, but he'd instead be opposed to it because of the implications it would have as a rallying point for the Rebellion at large.
I do think Zahn actually made Thrawn's ascension into the Empire make sense with the backstory he was given. I appreciate the Ascendancy novels for everything they give us, especially that we get the more nuanced depiction of Thrawn. He's a protagonist in a defendable light within that story—he's the good guy. And yet, even so, we see just how someone like him could rise to believing the system of the Empire is the way to go because he witnesses firsthand the way the Ascendancy is crumbling from within due to the political infighting and their restrictive rules when it comes to engagements and combat. Thrawn saves his people by defying their rules and customs.
The canon book Thrawn portrayal has just expanded upon his motivations. He is not opposed to heinous acts like that, but unlike the original Thrawn trilogy where we get this very narrow perspective on the matters (which I think indirectly can play into the idea of Thrawn doing such a thing just for the sake of cruelty), we see Thrawn in canon willing to do stuff like that, but it's more of a last resort, "I'll do what needs to be done" sort of ordeal. He says in the final Ascendancy book that he is willing to send an entire culture to death if it protects the Ascendancy; he says he's willing to sacrifice himself to save his people. The willingness to stoop to those levels in circumstances he deems it necessary, or the best option to achieve victory, is that self-sacrificing of himself and his ideals.
Thrawn comes from a place where his overarching goal, alongside protecting the Ascendancy, is to protect life, but he reaches a point where protecting life, to him, starts to come in conflict with that of protecting his people. It stems from his belief that the Empire is the key to the safest and most secure galaxy.
The rationalizing on his behalf is that he can re-install the imperial way of rule and do things right this time because there isn't a super weapon-hungry narcissist leading things this time around. But the issue, of course, is Thrawn just has no understanding of politics, and he doesn't understand that even the imperial system can inspire the same kind of infighting he saw within the Ascendancy.
Thrawn has lost a good amount of times at this point, in both the Legends and Canon stories we have of him, but many of those times it doesn't come out of his own failures. Well, indirectly, they do.
For example, in Season 3 of Rebels, Thrawn loses because Konstantine disobeyed him when they were in orbit and later, during the ground assault of Atollon, the Bendu. If it weren't for those two factors, Thrawn wins there, I believe.