I don’t believe bringing up Duel of the Fates is constructive since TRoS is all we have, but you can definitely have a villain be conflicted and still be the main antagonist. And that’s what he was in DotF, he even turned good in some drafts. Kylo’s whole arc in TFA and TLJ was about rising and becoming the villain. And having Kylo as the main villain would’ve definitely raised the personal stakes for our characters, Rey, Leia, Luke etc. Stakes aren’t defined by the ‘power level’ or the range of the villain. Starkiller Base destroying an entire system as opposed to a single planet didn’t make the stakes feel any higher than either Death Star, for example.