beta
I think he is from the future. In that case time he doesn't really care about the marriage as it is only contract and being forced. As they share information about their heat cycle sejin said that it will two more days but unexpectedly it came sooner. In Ido's eyes he must think that sejin was taking advantage of him so ignore him.
He treat him like he is not exist but sejin have to fulfill his father's wish so that time he did steal it the complete USB ( because of that he didn't complete the USB on the d-day) he thought that he got betrayed and raped him right there. In result his work get affected.
As everyone knew that ido doesn't really care about him his brother think that was opportunity and rape him too at that toilet (because of that he knew where to get there that time)
Besides, you guys know how he knew all the heat cycle I think I'm he might have take him forcefully as he think that was his duty or he thought that just how sejin try to seduce him ( this is why he feel guilty when sejin said that he was his first man)
After sejin can't take it anymore he fell into depression and killed himself using his gun ( he doesn't want him to go inside that room and even pulled out the bullet)