Not sure abt revenge, but I read that Zhenya will follow Taekjoo back to Korea and will even meet his mom. Zhenya also gives the Anastasia formula to Taekjoo and they become like a sweet couple in the side stories. Take it with a grain of salt since it's been so long since I read the spoiler forum.
Regret arc? I don't think so this will gonna happen. No regret arch, ML just surrender completely when he realised he can't live without him.
Spoiler!
After confining MC, It's looks like ML form a similar to Stockholm syndrome because they are only two in that place. Or he just really gets charmed by MC. So, when MC manage to escape, ML got so scared. It's kinda funny how scared he looks like in that scene. He can't live anymore without MC. When ML manage to find MC, he's doing everything to not cause a reason for MC to even think to run away again. So, he's kinda taking care of him or like under from MC. He even gave Anastasia to MC so MC can have power overhim. Like saying MC can use ML however he wants in exchange of staying with him. They became kinda sweet with each other. Although I'm not updated at the current volume, so I dunno if both of them fall in love with each other.
Oh, that's kind of "showing regret" I think. Thanks for letting me know. But yeah, it's unfortunate that it feels like stockholm syndrome.
Question for the novel readers: Does the ML ever genuinely cares for the MC or all he does is constantly rape or assault him sexually? I didn't fully read this yet btw, but I skimmed couple random chapters before... (the train stuff and such) Let me know if there is a regret-arc. Thank you~