The first child of God killed his lover
The second child of God killed her son
The third child of God killed his mother
These three children of God sinned by killing each other so God cursed their rebirths with dark/curse elements. The curse is to make the same mistake every rebirth, remember their past lives, and live in suffering every life. Anyone who has a dark element will remember their past life. Those white pills suppress the darkness/memories and also kill. 1st son has big brains and manages to make a loophole in the curse cycle while still following the curse.
The first child of God was cursed by God to always kill his lover aka female lead/ second child of God, he will always remember what sin he did in each past life and will kill his lover every life.
The story starts with the 1st son of God being a normal holy son in the church who was a lover of the then still holy saintess/ female lead/ 2nd child of God. In that previous life, they both haven't been cursed by God yet. Saintess fell out of love with 1st. So 1st killed the female lead/ saintess since that's how life works for these sinners.
The second god's child, the daughter of god, the female lead/ original saintess is cursed by God to kill her child. The author tries to wash off her original sin by not mentioning it. He never talks about her killing her kid but you can easily infer since 3rd/able is supposed to die, she's a sinner. Anyway, the saintess/2nd was killed by 1st when she fell out of love with him. She lives a life of sin and kills her son/3rd somewhere down the line and god cursed her lol. 3rd killed her back in some life and got cursed too. Anyway, she was reborn as the woman who gave birth to 3rd/Able the red-eyed little kid she 'stans' with the young duke, she got killed again lol. It's not made clear but she is probably the reason the kid is supposed to die from magic corrosion or whatever. She was reborn into Korea and was killed by 1st again lol, then she was reborn into the body of the current story white-haired girl who has just gotten killed by 1st again using those while murdering pills from the church. This gives the characters a chance to end the cycle of rebirth and regret.
She is cursed by God to kill her son and regrets it since she's a sinner and God knows how to punish his kids well.
The third son of God is the one who wrote the story FMC is currently going through, they don't write too much about this guy until the side story but he keeps killing his mom. Anyway she's currently in a coma and braindead but he has her on life support. Eventually when the story ends and the female lead ends the punishment the mom wakes up from being brain dead so he isn't punished anymore either.
Also since the female lead finally doesn't kill her son and kills herself instead so the cure ends, 1st doesn't technically didn't kill his lover since she was reborn 2nd gods child, since they fix their shit the 3rd child kept his mom on life support, and the curse breaks the chain of them killing each other so his mom wakes.
Anyway, the story is all kinds of esoteric bullshit but it was interesting and cliche. I wish the Koreans would stop basing their story on the same template but whatever I guess I like reading the same thing over and over lol.