The description and story plot given was a *tad* bit too specific so i’m not sure how this’ll work out but oh my god. you HAVE to read this: https://www.mangago.me/read-manga/olgami/
it has the slow burn (VERY MUCH SO), lots of murder, twisted romance , unhinged, angst, denial, touching (but not smut), longing, and basically everything red flags but we as readers interpret as green

Is there any mystery or thriller manhwas (preferably modern) where the male lead is a killer and the mc wants to get into the bottom of it. He isn’t misunderstood or anything, just unhinged and starts to develop feelings for the girl, but he keeps denying it and brushing it off. I want ANGST, DENIAL, SLOW BURN, LONGING AND A BIT OF SUBTLE TOUCHING. Im thinking something like ‘The guy upstairs’ BUT IM AFRAID ITS NOT GOING IN THAT DIRECTION