how i interpreted it was that the grandmother and her twin brother were always fated to be together. therefore, the punishment the grandmother would endure for her built up hatred for her brother would be shared with him. in the end god decided to take both twins, mix them together and then split them in half. i might be totally wrong and anyone pls feel free to correct me, but it seems like both of their spirits when into hwarim and bonggil. neither fully malice or fully good- a healthy mix of both. so now hwarim and bonggil are kinda like two halves of a whole.
I got the same interpretation
Whatever happened at the end part of ch25 I didn't get it.