
Past life Ido is a jerk who thinks sejin married him for his company/money so he could get pregnant and have an heir? He misunderstands that sejin is emitting pheromones on purpose to seduce him. Now it's been awhile since I have read the novel but sejin is mistreated, raped by Ido, then they kinda grow closer because he realized that sejin isn't in it for the money.
Then sejin steals the..usb (in this life, in the next life or the new timeline he doesn't as we read already) so Ido feels betrayed? Somewhere along the line Sejin is raped by Ido's older brother - another misunderstanding created by Kwon Ido himself (they don't communicate, it's frustrating). When he asks Sejin about it, Sejin says that he wanted it and that it was consensual. He is stressed and traumatized and hates Ido at this point because he has been putting up with his bs for so long, and his family too, blames their company's failures on him (which is why Ido made him quit his job at Haesin immediately as they got engaged in the next timeline).
Anywho then sejin finds out he is pregnant. Ido, jealous and mad rapes sejin right after his brother so sejin doesn't know who's child he is carrying. So he heads into the study and shoots himself. Ido, meanwhile finds out everything, runs to save sejin. Couldn't. Sejin dies in his arms.
(The sequence could be a little jumbled up, sorry about that)

Sensitive content be aware...
The grape happen during Ido's grandfather funeral. It was for 3 days i think. Kwon ijung( ido' bastard brother) misunderstood ido when ido said he have a gift for ijung at ido's mansion. Ijung thought that Sejin in heat was the gift and grape him. It was brutal and bruise. Then sejin learned that ido kinda sold him to be grape by ijung( which is a major misunderstanding).
After that happen Sejin called Mr. Kim his secretary to pick him up and bring him back to his own home (aka running away from the abuse). After like 3 days or so Ido came back to his mansion and looked for Sejin everywhere but cant find him. So, he check his CCTV( this one not sure) and trance Sejin back to sejin's home. There he found sejin passed out with sleeping pills around him and thought he was trying to kill himself.
Unforunate for Sejin because he was grape he had severe trauma and was using the sleeping pills to sleep and forget it. Then ido minsunderstood that sejin want to kill himself throught the pills. At this time ido was going throug his Rut and he was ansty and angry with sejin. They fight and Sejin said he want a divorce. Ido became so angry that he grape

A bit correction on the grape part...
Spoiler & Sensitive content be aware...
The grape happen during Ido's grandfather funeral. It was for 3 days i think. Kwon ijung( ido' bastard brother) misunderstood ido when ido said he have a gift for ijung at ido's mansion. Ijung thought that Sejin in heat was the gift and grape him. It was brutal and bruise. Then sejin learned that ido kinda sold him to be grape by ijung( which is a major misunderstanding).
After that happen Sejin called Mr. Kim his secretary to pick him up and bring him back to his own home (aka running away from the abuse). After like 3 days or so Ido came back to his mansion and looked for Sejin everywhere but cant find him. So, he check his CCTV( this one not sure) and trance Sejin back to sejin's home. There he found sejin passed out with sleeping pills around him and thought he was trying to kill himself.
Unforunate for Sejin because he was grape he had severe trauma and was using the sleeping pills to sleep and forget it. Then ido minsunderstood that sejin want to kill himself throught the pills. At this time ido was going throug his Rut and he was ansty and angry with sejin. They fight and Sejin said he want a divorce. Ido became so angry that he grape Sejin with knot and they bonded. In this story after a couple bonded both of them will relive each other memories and ido was overwelmed experiencing Sejin entire memory from the kid sejin to recently.
So yeah this part maybe a long time coming for this season. Lets us hold hand and hope our couple able to be strong after the flashback season over.

A bit correction on the grape part...
Spoiler & Sensitive content be aware...
The grape happen during Ido's grandfather funeral. It was for 3 days i think. Kwon ijung( ido' bastard brother) misunderstood ido when ido said he have a gift for ijung at ido's mansion. Ijung thought that Sejin in heat was the gift and grape him. It was brutal and bruise. Then sejin learned that ido kinda sold him to be grape by ijung( which is a major misunderstanding).
After that happen Sejin called Mr. Kim his secretary to pick him up and bring him back to his own home (aka running away from the abuse). After like 3 days or so Ido came back to his mansion and looked for Sejin everywhere but cant find him. So, he check his CCTV( this one not sure) and trance Sejin back to sejin's home. There he found sejin passed out with sleeping pills around him and thought he was trying to kill himself.
Unforunate for Sejin because he was grape he had severe trauma and was using the sleeping pills to sleep and forget it. Then ido minsunderstood that sejin want to kill himself throught the pills. At this time ido was going throug his Rut and he was ansty and angry with sejin. They fight and Sejin said he want a divorce. Ido became so angry that he grape Sejin with knot and they bonded. In this story after a couple bonded both of them will relive each other memories and ido was overwelmed experiencing Sejin entire memory from the kid sejin to recently.
So yeah this part maybe a long time coming for this season. Lets us hold hand and hope our couple able to be strong after the flashback season over.
SOMEONE SPOIL ME PLEASE