Responses
In the novel, the last part tells it from Kwon Idos point of view, and you understand a little more.
After Sejins betrayal, he just was blinded by rage. Ido never saw himself loving anyone, and fell in love with Sejin. There were no security cameras in Sejins room, so he rationalized it in his mind, and Sejin didn't explain, and egged him on.
Not that Sejin was wrong, he just was betrayed himself with the grape.
I gotta give it to KID - lol using his initials is diabolical! Ido gave the best response. There's nothing he can say that will give the best explanation to his behavior. Even I'm confused with how he handled things. Like why did he rape Sejin, again, after Ijung, when it was obvious he was angry at Ijung and he handled his brother after what he did. Explaining is not making excuses, just explaining would suffice but there's nothing to explain! So yeah, just kept saying sorry is really the best thing he did.