My guess is that he somehow lost it for a while (heat, maybe a drug or whatever that left him in a state of pure instinct rather than logic) and raped flower lad, forgot about it, and now his memory is completely wiped out.
ok so i read the raws so SPOILER!
flower dude has always wanted to kill dragon dude ever since he was young (idk why can't read mandarin), but their last encounter before flower dude disappears for a century before they met again at present day was when the thing happened:
flower dude dressed up as a female dancer/bed warmer(?) -> dragon dude already knew it was him so he chose the dude to come to his bedroom -> but before they met dragon dude met a demon gal (?) that may drug him with aphrodisiac or some sort cuz my man seems hot and bothered -> enters flower dude how came to assassinate him -> ends up getting rape but dragon dude doesn't rmb cuz he's on drugs -> flower dude ran away and then they met each other again with aoao
so the big bro has met flower dude ever since he was a child according to this chapter http://www.mangago.zone/read-manga/aoao_waiting_to_be_eaten_caught/hs/iur_chapter-49/pg-1/, the "first" encounter (when flower dude try to shoot an arrow as the dragon) is before the sex or after? i'm so confused by the timeline since when they had sex (from the flashback scenes), the flower dude was an adult but from big dragon's memory, their first meeting was when the flower dude was a kid