It's a whole conspiracy between the important people in the village and their children. Obviously Go JeongWoo didn't do it - the girl he was interested in had an older, already married lover. She was killed and then some of GJW's friends AND their parents also covered it up. The other girl - BoYeong - was raped and NaGyeom made the boys that were involved kill her too and dump her in a reservoir (well, I guess she's the one who's bones are resurfacing now). NaGyeom wanted GJW for herself, she never thought he'd be convicted just because of circumstancial evidence, but he got convicted anyway and she still doesn't want to let him go (that's why she's still visiting him in prison and getting in his business). Her price for staying quiet was getting inside the TV-drama world, but she wants GJW too and is delusional she'll still get him.
SuHo was also a witness, and even if he's (high-functioning) autistic, he wanted to tell, so now he's being under close monitoring by one of the actual original killers, and threatened with being put in an institution if he ever tells anything. His drawings are his way of telling things without telling them out loud.
The original motivation had to do with lots of money and land rights (that GJW would also have inherited a major part of, if things had gone as intended, but making him into a killer while saving some others is also how they got rid of him in the first place).
Ha Seol is going to start unearthing things (well, so to speak) and also GJW won't take it laying down. Na SangCheol (the cop) will eventually start catching on).
That is, if they follow the general plot of the novel. Things can be changed in an adaptation.
