Yeah, I'm not understanding why Sunghoon is getting so much flak. At the end of the day, it's an unfinished business between Rangi and Hwagun, with the other bitch interfering because she's insecure as fuck. MC is forced into their affair no thanks to his parents, and though he did decide to stay, this Sae girl is really unreasonable towards him, as well as abusive (lots of people are abusive towards MC, mainly girls, because it's funny when boys get beaten down by girls right?).
MC from what we know has lived an ordinary life in a peaceful environment (though it's implied his mother is kind hardcore). Presumably, Sunghoon gets thrown into a violent situation, gets his life threatened, then saw his childhood friend and crush beaten into bloody pulp, most people in his situation snap. Sae is quick to always bash him to think, but she sure doesn't think about him at all. She's biased as fuck, thinking that a 15-17 human child should be as intelligent/experienced as her or something, when she's pretty old phantom, and her master even older yet as mature as a child. Heck, Sunghoon wouldn't be involved if Rangi didn't fail to learn love in her long life well before he enters in the picture.