Responses
Maybe you forgot, Sejin asked for the bound. As he should, tho. In current timline their relationship is so good that Sejin who doesn't have the memory of the past believe they're really a lover. It is only natural in ABO world asked your lover for a bound that tie the relationship together. Ido did try to avoid it, but it made Sejin anxious. Thus come the scene of them bounding in that same exact office which ironically killed himself.
About staying, Ido feels he doesn't deserve Sejin. So it is only right for him, as he said, let Sejin go. COWARDLY of him. He didn't let Sejin to decide for himself. He thought, his 'sacrifice' is the best for Sejin.
I still don't understand. Why the hell would he mark him again the second time and still not have the backbone to stay with him? He doesn't deserve redemption!