but why did he change so suddenly tho (╯°Д °)╯╧╧

He didn’t suddenly change tho he’s like this from the very beginning. But he dare to hurt ZS because he thinks ZS doesn’t know anything about those mistresses outside. He have sense of guilt but doesn’t stop, a trash. And when he know ZS knew everything from the start he all worked up and scared for what he did will make ZS loathe him, leave him so he decided to break up with that boy toy the next day. If you read the novel version he actually is the scum who love but doesn’t know how to cherish what he have until everything is too late.