Anything (shoujo, shounen, bl, a recommendation list etc) where MC is a canon fodder/villain and the og!protagonist is either/and/or: gradually blackening revealing their already blackened nature despised/not as loved as they would be in og!novel because of their obliviously ignorant, hurtful or ambiguous behaviour genuinely normal until they sabotaged themselves and missed their chance at peace and happiness because they were focused on destroying MC or anything where they don't end up in their og!glory, even if slightly off Because/despite of MC's change due to rebirth/reincarnation/transmigration/possessed by a "host" in a QT novel/etc. An example, Rebirth of the Evil Mother-in-Law I've seen a lot of og!FLs-become-antagonists (and enjoy it very much and anticipate them very much) but don't forget to rec some og!MLs-become-antagonists if you know them too.
I Just Want to Freeload on Your Luck should fit the bill but translation is slow and we aren't that far in the story yet. (can't do a proper link since I don't have 5 total things posted)
Juder have this series called Immortal Regis. The MC become the main antagonist in his later series, cavalier of abyss. It got great story and great art.