(Novel should be a quick transmigration) The ML was a yandere priest (?) for the FL saint later the protagonist and could see the sins of a person, which caused him to go crazy in the original novel and kill everyone. The sinless protagonist was transmigrated into the body of a vampire duke/lord and the ML couldn’t touch the protagonist (holy power or something) because it would kill the protagonist. Protagonist ends up dying, saving ML and then goes into the next world.
Pretty sure it’s a web novel that’s not on here, but it should be “Quick transmigration: Homewrecker system” arc 1