Yes, this is a tricky one. Of course Larian intentionally wrote the companions/origins and Durge specifically so they could go in different directions, but I confess it's not something I've generally considered when constructing my own characters. But now the question's been asked, I'm pondering options for characters that could develop quite differently depending on the rolls of some dice and what those key rolls might be!
Of the Tavs I've played so far.
- My first tiefling Wild Sorceror/Rogue was more chaotic than good, but believed in freedom and hated authority and those who exercised power over others so the specific flavour of evil on offer was never going to be tempting to her. I think for her the crux point was not visiting the Mountain Pass but instead travelling through the Underdark, which meant that Voss didn't show up in camp and she didn't have enough influence with Lae'zel to get her to renounce Vlaakith. That meant that she had no knowledge of a Gith resistance or even that anyone else knew that Vlaakith had imprisoned Orpheus and wasn't actually Gith's chosen heir, and therefore believed Orpheus was the only hope of freedom for the githyanki. Had she known Voss was fomenting rebelliion, and had Lae'zel been part of it, she may have let Orpheus become a mindflayer to defeat the Netherbrain and she might have lived happily ever after (though not with Wyll who showed himself not to know her *at all* by proposing marriage after less than a couple of months - given a couple of decades or even a couple of years she might have come round to the idea!). As it was, she felt she needed to make the sacrifice and ended up taking her own life rather than live as a mindflayer.
- My second Jack of all Trades run was probably only going to go one way. My halfling's prime driver was knowledge, and while she might have been curious as to what controlling the brain felt like she had zero interest in domination. She did lose a roll to try to avoid consuming the Astal Tadpole, and had she not become half-illithid she may not have taken the final step, but even if she'd succeeded I think it would only have been a matter of time until curiosity got the better of her.
- My third run with a devotee of Eilistraee was never going to have an "evil" ending either, though I guess it could have been a bit darker if things had gone differently with Astarion. I'd intended them to romance him, but either through too low approval or making wrong choices, Astarion wasn't interested so I ended up roleplaying it as unrequited love. And unlike in my first two runs, Astarion actually wanted to ascend, and being unsure about releasing 7000 spawn my character probably would have supported him rather than kill him had they not managed to persuade him out of the idea. But I'm not sure exactly how that would have affected the outcome.
- My monk/cleric of Ilmater is still at the beginning of act 2, but agan I don't really see any big turning points that could lead him to want to dominate the brain. He's all about the simple pleasures and peaceful life, so brain domination isn't slightly tempting. I'm hoping that he'll be able to settle down happily with Halsin and raise orphans and heal the shadow-cursed lands, but I guess it's still possible that Halsin won't find Thaniel in which case his HEA may not come and he'll probably just drink himself into oblivion once it's all over - or maybe without love and hope to tie him to life he'd make the sacrifice and become a mindflayer even at the cost of his soul.
- My divination wizard/cleric of Mystra is a star elf whose motivations include a desire to find a way to protect her home realm from invading aberrations. She's probably going to be the most conflicted, given the potential power of the crown to help in that fight, her devotion to her goddess and her growing relationship with Gale are all in some tension.
Thanks for asking the question! As you can see, it got me thinking

Though admittedly more about my characters as origins than companions. I'll have to ponder that some more.