Bum fell for Sangwoo because Sangwoo saved him when he was being raped in the military, if i remember correctly. This caused Bum to get attached and soon obsessessed with Sangwoo because of his kindness at that time. I'm annoyed that Sangwoo doesn't even remember the first time they actually met, and hopefully at least Bum remembers correctly. ヽ(`Д´)ノ
An ex-lover? I am intrigued