mystery webtoon
preferably completed or has a lot of chapters
don't mind bl/gl/het
Mystery as supernatural? if yes those 2 are pretty good https://www.mangago.me/read-manga/the_imposter/
https://www.mangago.me/read-manga/survive_romance/
Either the author is being sneaky and making it so lovey dovey to give the biggest twist in the end with the long-awaited revenge. Or they're making everything so overly cute and unserious so that people slowly forget that the man is a murderer and root for them instead.