I have a feeling that the scene with "rain" or whoever it was in the early chapters was the true culprit of that incident, and it was passed off onto Ryeong's brother because he had gone insane due to the twins' mental manipulation and or qi deviation, and had no way of standing up for himself. The line about "even the dog" that Eulran mentioned reminded me of how he killed everyone in that village and ended with the dog during that sequence, so I feel like we'll finally get the reason behind why that scene was included.
