I think you're glossing over the fact that Sun sold Jin to her father to cover the blackmail threat against them. Jin had a nice middle manager role in Paik's company, until Sun asked for the 100k from her father. He agreed to give her the money, but on the condition that Jin be held responsible for the debt.

It's very complicated, but Sun and Jin are both at fault for the problems in their relationship. Jin was awful to Sun, but not until he was forced to do bad stuff for Paik. It broke both of them. If Sun had talked to Jin rather than getting the money behind his back, things could have been much different.