
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...
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.
I agree. Mari was really hypocritical in her action, I thought.