I guess the plot twist is that all of them the culprits lol ╮(￣▽￣)╭

FR, since like the early chaps, i was like, it's for sure ALL of them and they are taking turns, either they all know about it or they somehow managed to jump him on separate days and not get caught, it's seriously giving "Murder on the orient express" where everyone is guilty but they help each other with alibis, if we're wrong this would be the plot twist of webtoon history