My theory is: Just like in 'You who came from the stars', the in-law here (of course that evil witch stepmom of the ML) of the FL in the past tried to set up a 'suicide' by drowning her in the well so she could request from the government the so-called 'yeolnyeo' or 'virtuous woman' award (as can be seen in the grave in ep 4) which entails many benefits and rewards. That Joseon award is given to the wife (dead by committing suicide as if she followed her husband in the afterlife) who led an exemplary life by remaining loyal to their late husbands.
So far, I am really loving this show but quite sad and disappointed to see how unfair it is for GP Gyeseong's intelligence and capability to be lockep up just to keep his secret from getting out.