That is just one of his pyscho way of thinking manifesting, he kind of have it in him.
I've thought about this throughout most of the manga, but they never really tell you. He could be one of those people who feel the need to destroy what they love. I do think over the course of the manga that Yule starts to regret it, it is shown a lot towards the end. I think losing Kyung Soo opened his eyes and he wanted to treasure him. Yule has some major issues, but those two are bonded together. I wish there was another few chapters to show us that they could find happiness together. I thought we were owed a better ending that this. Too many loose ends in my opinion.
I have a theory : I know it's common for people who suffer from borderline disorder to "test" their relatives. They makes them suffer in order to confirm that they won't be abandoned that easily. I don't think Yule suffer from borderline disorder but he could just share this one symptom
Why did Jule set up the gang rape? Did something happen or is he just fucked up? I only rwad the first few chapters and some of the ending ones and I just want to know his logic.