
So Mari can treat Junyoung any way she likes and drag him into all sorts of stuff from her dating habits and it's fine but he makes one mistake (not even a real one) and he's scum of the Earth? I would understand if that were actually her boyfriend, but she knows that this was all a misunderstanding. Only a miserable, shallow person would devote their life to making someone else miserable over a misunderstanding. No matter what arbitrary boundary she puts on her relationships, she can't pretend that she never destroyed someone's family and broke people's hearts and trust. At least Junyoung had the decency to feel bad, but she can't even manage that much. It's clear based on where she chose to meet that she has no intention of actually working this out. Junyoung and Sihoon would be better off just cutting her off until she decides to stop acting like a bratty child.

But why would his mother leave and not take him too? She knows what kind of person his father is and that, as far as she knew, he could very well wind up dead or being used like she was. So why would she leave him behind?

She is one of that women who wanted to start new without the husband and even with the child. The son maybe would even remember her to much about the shitty ex. Her love for this child is not big enough or she loves herself more then her baby. In real life it's often the same. Just the mother of two of my half-brothers did the same. She also said she will come back and didn't pick my brothers up from the kindergarten. She never ever take care of them...
I honestly don't think the wife deserved a happy ending. She didn't care about anyone but herself and, as a result, did horrible things to people with no remorse (cheating in the first place, using Yeon Woo and getting him sexually assaulted and then killed as a result, trying to kill Ha-ri by pushing her in front of that bullet). You can say she cared about her husband, but she even said he wasn't allowed to be happy without her so that isn't entirely true, either. And then she just planned to walk off and live happily ever after without taking any responsibility for anything?
At least Ha-jin, who also has a long history of being abused and assaulted by that crazy president (and probably has a troubled past if he became a hit man), had a change of heart but Si-yeon just kept getting worse and worse. She didn't even care when Yeon Woo was supposedly shot to death in an elevator. She was just glad no one could back up Ha-ri about the affair anymore. She hasn't learned anything and never felt sorry for any of the people she hurt. I'm in the tiny minority of people who hated her through the whole thing and was glad when she wound up dead.
I hated the wife soooooo much. She only cared about herself. I hate that she ended up being super manipulative. I liked the ending. I feel like she got what was coming for her.