To summarize: His mother adopted him as replacement for the "flawed" older brother ╥﹏╥ She doesn't like him doing dancing but go for business like her while the father want him to takeover his military position. But, the mother's family is more powerful, so she'll get what she wants. It's like she is "investing" on the adopted cuz the older one can't or was useless to her. She is also just "evaluating" him if he is fit to takeover her or is useful, thus the "don't make me regret it" words from her. Which triggered her karma.
He realized what kind of mother she is after that scene, he probably decided to drag her under to protect his current life and loved ones - his brother and Baek. I'm guessing he collected evidences during the passed 10 years to reveal her as well as get back to her (given his personality and temper) by ruining what she finds more important than them... her own reputation. ヾ(☆▽☆)
Am I high but I get but also don't get the situation between sinjae and his mother? At first I didn't get what he meant by celebrating the wrong b'day and turns out (SPOILER ALERT) he's adopted. Is his mother disappointed because he chose a dancing course and that he's useless? I'm gona be pretty dumb to ask but why is his mother in jail? Abuse or smth?