
Well, the MC have known about it for a long time
--> check the scene when he was only a high schooler when he saw the church burning and the adoptive mother's car rushing out of the town.
When you doubt a person , you'll never indulge yourself to her love, affection, kindness. Cause no matter what you know in the back of your mind that she is the culprit.
He only acted oblivious about it while trying to find more evidence along the way.

Cha knows about her being the murderer since he was young but he probably has no evidence to convict her after collecting evidence he told her I know it's stupid to tell her rather than doing action , even if she adopted him we probably would know Cha is related to Haesoo mom even before getting adopted so she had her own purpose to adopt Cha despite having son of same age.
I am sitting here wondering if him asking his adoptive mom to turn herself in sounds a bit unrealistic, like yea she might be the murderer but she loves u treats u like her son and even better than how she treats her actual son, I think it would have sounded more reasonable if he tried to cover on her, in anyway no one can blame him for it, he doesn't always have to be Mr perfect who willingly sponsor his abuser and still plan on continuing to do so.