Well all three men, Yule, Geon and Dong-hyuk rape him and are extremely selfish, so all of the "what if" scenarios are pretty depressing. With Yule, there was a lot of psychological abuse that could have been a motivation for him to return, Kyung-soo also had more emotional ties with Yule despite their relationship failing, he still loved him in a sense despite everything. All of the men who Kyung-soo has had sex with, have raped him, and he considered himself to be in a relationship with two of them. He maybe feels unable to move on and stay single, or find a normal partner, considering he had it drummed into him that he is the one to blame, that he corrupts people and exposes them to harm. Kyung-soo isn't going to be romantic and trustful of Yule, it's Yule who is now trying a new setting in his manual on how to act in order to try and keep his obsession beside him.
I guess what I said was kind of a rhetorical question for me cause I've gone through the same thing. Always going back to the person who hurts us most RIP
What's the point of him going back to Yule if he was just gonna treat him like ass. I was gonna say it makes both of them miserably but the smile Yule has makes me think his stupid self is getting better.(づ￣ ³￣)づ