@nikonekonya My theory is they rather adopted her because they thought she was harmless and wanted to be the "perfect family". Which is why the mom was trying to cover up the "murder" and the dad's reaction to Soo-yeon. But anything could be possible~
@PostImpressionism I have a feeling it's related to Strangers from Hell, so maybe watch that drama or read the webtoon and it might help understand this story better or help you get into it :)