Been mulling over this line from the Matthew Stover book for a while: "I think," Obi-Wan said carefully, "that abstractions like peace don't mean much to him. He's loyal to people, not to principles. And he expects loyalty in return.

I think it's a little bit more complicated than that. Like, even with Sidious he had to slowly be walked into accepting his arguments, that everything Sidious did was in justifications for the greater good. Like, sure, the entire time he could be walking around with alarm bells in his head at the stuff he's doing, but ultimately he needs a cover to believe what he's doing is right. He's not the type of person who can gleefully enact evil for its own sake.

The entire time we see him as Darth Vader, we never really see him truly happy. Maybe he has brief moments of respite or triumph, but ultimately he's miserable, he's not thriving off of the evil he does, but rather sees it as a chore to ensure the proliferation of his ideals.

What I'm trying to say is, Anakin is not merely loyal to people in disregard to ideals. He's loyal to people above ideals, sure, but his loyalty to a person is also contingent on them acting in accordance with shared ideals (loyalty and integrity in the case of Obi-wan and Padme, or adherence to the jedi code and fundamental principles of justice as with Mace) or at least being able to justify their deviation from it as with Palpatine. When a person deviates from those ideals without what he considers sufficient justification (like Obi-wan's refusal to let him help his mother), that's when he turns face.

I think a person's integrity is particularly important to Anakin. He lives in fear and uncertainty, so the thing most important him about a person is that he needs to know what they are about, through and through. When he gets put into a position where people say one thing but do another, like Mace with the Jedi code but being ok with executing Palpatine on the spot or the Jedi helping the Hutts, that's when he gets conflicted and doubtful. And that's partly been Palpatine's strategy in pulling him away from Obi-wan and Padme, by artificially breaking their appearance of integrity to him. I think that's partly why Luke's strategy was particularly effective with him. Cause Luke was consistent to the point of death in his messaging, it gave Anakin the basis to trust Luke's claims beyond even what's been hammered into his head for the last two decades. Palpatine's an outlier in this of course, but Palpatine's so good with his messaging and manipulation, that ultimately, Vader can't get ahold of the logical footing needed to conclusively call out his bullshit.

So ultimately, I guess what I'm saying is that it's not so much that Anakin doesn’t care about ideals, it's that his understanding of them is so surface level that he ultimately needs them to be rooted in a person to help him navigate the nuances of following them. Thus, he's beholden to people more than ideals because ideals are not tangible and accessible to him without the filter of a person. In turn, he then requires the people who filter these ideas for him to be unwaveringly consistent, or at least able to appear so in a way that does not trip up all his worst anxieties about abandonnent, loss, and indeed, being coerced (the important thing about this last point is that he's needs to not feel coerced because he doesn’t have language and training to spot actual coercion).

I'm not wholly satisfied with this explanation and think there's more to pick at here that I will need to come back to, but yeah. This is my take on his apparent flip flopping of ideology. I think, in his head, ultimately he's not. It's just that what those things mean to him and how they translate to real life is heavily dependent on who he's using as a filter. At the heart of it, I think he just doesn’t have enough faith in his own interpretations on his ideals to not be easily swayed by the next most convincing argument. Grooming will do that to you, I guess.