
Uhhh this spoiler is totally wrong and that's not what happened.
He was brainwashed until Jihan interfered, and he regained his mind, so no, he didn't choose to leave Seah behind. It was Ito Shizuru that made him leave her because his love for hos daughter interfered with her mind control over him.
I say this as someone that is reading the novel.

Yes, he met her during his grief period.
And the last chapters did show she had to use "Obsession" to keep him in control. Her ability is one to edit people's status windows, she can also edit appearances, edit the description and function of items and etc. It's how she puts the status abnormalities in the players.
“Obedience” and the other one she usually used to control the other people she controls don't work on him, and she has to be by his side all the time because her control over him fades if she is away from him. He starts to question things, so she has to keep renewing the "Obsession" trait so he is obsessed with her. It's not voluntary.
If here they show what he did when he finally regained his mind, I think people will get him. In the novel it was a reeally nice. But let's see how they will spin the arc.

Spoilers
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To make it simpler, our MC soon will become a National Player and during one of his matches against Japan he will try to free Yoo Sejin. He will succed partially in freeing him from mindcontrol and Sejin will start to fight off Shizuru control, and one way he found to do that was blinding himself.
He basically destroys Japan's player center, kidnap the National Players and start to threaten them to give him Shizuru, so he can kill her. But she escapes and Yoo Sejin starts to follow her trying to kill her, and because of the "Obssession" she put on him he is able to track her down. Jihan goes to Japan to look for her with him and both of them do a manhunt and finally capture and kill her. It was actually quite an amazing scene.
This will probably take some chapters because jihan still has to become a National player and he still needs to get his hands on some of the World Tree leaves (World Tree leaves, branches and etc can cure Status abnormalities).
After that, he is taken home by Jihan and he starts to go into a process of reintegrating himself in Korea. Him and Jihan start working a lot together actually.
About the previous time-line, during one of the matches he is Bethan by an Elf that had a weapon made of a branch of the World Tree, and he was freed from Shizuru control then, it wasn't clear if he killed her there, but the fact he disappeared it was because he was in despair about what happened to Seah, and he went mad with grief and anger.
It will be show later that Jihan's sister is actually alive, and it was taken as part of Void because of her sacrifice, becoming the Witch of the Void, that is in charge of seeing the end of world (earth). Sejin finds her and as a way to be punished and atone for his daughter's death and misery he becomes a monster that destroyed China in the previous timeline.
I hope this helps!
Am I the only sympathetic to the dad? He got mind washed and he didn’t choose to abandon his child. I hope she and our MC can save the dad ASAP! It’s depressing to know you have no choice in life but I can’t wait for him to pay them back for taking his freedom and time with his child that needed him. That B!tch deserves her ass to be destroyed slowly and painful.