He was the only one that knew what it felt like to not want to feel.  When she had nowhere to go, no one to turn to, he was there even when she didn't want him to be.  So he helped her in his own way, even if it would kill her in the end, but she didn't care as long as she died the way she wanted to.  To Kang Seulgi, death by Kim Jongin didn't sound too bad.