the son was dying. he couldn't get a heart. father talked to kim joon to get him a heart, kim joon promised to get a heart for him by probably putting him first in the list of the people who would get a transplant. father promises kim joon he would do anything for him, kim joon had a vendetta against suho due to an old case that got suho and the police guy fired and he wanted to teach suho a lesson. so kim joon wanted to kidnap gunwoo, and, perhaps gunwoo died accidentally and seonyul's father had to take the blame. for the heart, and also because kim joon promised he would get his punishment suspended. mom found out about who was behind it and that's why she got into the "accident"
I can give you the "spoiler" if you want. Do you wanna know?