I'm wondering if there's any novels where the MC actually changes the villain completely to become a better person. I've read Transmigrating into a Mob Character to Rehabilitate the Villain Plan and The Reader and Protagonist Definitely Have to Be in True Love. Both of them, the MC is a good and kind character, who tries to make the villain a better person. But fails. Once they fall in love with the villain, they start to build a bubble around themselves and completely ignore how the villain murders and tortures people. By feigning ignorance they end up being an enabler, and thus, part of the problem instead. I don't know how you could love someone who slaughters whole sects without blinking, and still have "good guy" in your resume. Is there a novel where the MC is stricter and actually succeeds in changing the villain instead of ignoring the problem?
I don't know about a novel, but your request reminds me a lot of the mahou shoujo genre. Where they would fight this big bad villain, and by end of the arc, MC finds out they are actually good inside and said villains (or part of them) join the MC. Then 5 seasons later it is loaded with villains turned good cast lol (Though shounen does that too sometimes, just its not a common)
Its common thing, it was called defeat means friendship .... let me find a link for you https://tvtropes.org/pmwiki/pmwiki.php/Main/DefeatMeansFriendship
Villain doing 180 is one thing, but the 'good guys' doing a 180 from kill the villain to forgive the villain
https://www.novelupdates.com/series/youve-got-the-wrong-house-villain/ https://www.novelupdates.com/series/methods-to-save-the-villain-who-was-abandoned-by-the-heroine/ https://www.novelupdates.com/series/akuyaku-reijo-nanode-rasubosu-o-katte-mimashita/ https://www.novelupdates.com/series/if-i-happened-to-tame-my-brother-well/ https://www.novelupdates.com/series/the-villains-sister-suffers-today/