I've only read this upto chapter 10 and I have a question. WHY THE F*CK COULDN'T KYUNGSOO ...

Because this is yaoi, police and STI's don't exist! Kyung-soo was a victim of abuse. His first boyfriend, who he loved and thought he could trust emotionally abused him, before allowing his friends to rape him. Why didn't he go to the police after the first rape? Because going to the police involves physical examinations which can be traumatic, it involves reliving the abuse through interviews, it means having to go to court and be accused of being a liar, and potentially seeing your abusers walk free. Kyung-soo in response, dropped out of university, he moved homes, he moved jobs and changed all of his contact details, he thought he was moving on, but was still suffering from the trauma. Kyung-soo's feelings towards Yule were complex, as you would expect, he despised Yule, but could not ignore that he did used to love him. Ideally both brothers Geon and Yule, as well as Dong-hyuk would face justice for their crimes, but they didn't.

He could've gone to the police to report Yule for Dong-hyuk's kidnapping.
(But then the manwha probably wouldn't have reached upto 70 chapters)

I wont spoil things, but as well as what was mentioned in the previous post, Kyung-soo was made to feel that he was to blame for what happened with Dong-hyuk, he feels like he made Dong-hyuk into someone abusive, and who exposed Dong-hyuk to violence. In the last scene, Yule manipulates him by saying they have this shared fault of destroying good people. Kyung-soo wasn't thinking logically, he internalised the abuse and thought that he could protect Dong-hyuk from from Yule, and also himself, only if he cut off contact and went back with Yule. There has already been a lot of discussion about the dynamics of all the characters relationship in the comments if you scroll back. The gist of the manhwa is Kyung-soo and Yule's relationship, in it's entirety, and the affect this has had on others, as even when they are apart, both men are traumatised by their partnership and follow this onto their lives outside of this, as seen in the latest "what if' chapters especially.

Kyung Too couldn't ask for help from the police or really, any form of law enforcement, because as @clouds stated, he was traumatised. In addition, the author decided to bless Yule and Geon (the brother) with an infinite amount of financial stability and influence over the community. When you reach the end of this webtoon/manhwa, you could see that, unfortunately "justice doesn't prevail" and you have to move on to the best that you could...

╥﹏╥ this manhwa did a satisfactory and realistic portrayal of Kyung Soo and his internalizations of self-blame.

Please correct me if I am wrong, but does the last scene refer to when they were in the car? I thought it was Kyung Soo that voiced and reminded Yule that because they both hurt a good (haha, questionable) person, they would be unhappy together. Yule just agreed, so I did not see any manipulation on his part. If you meant that due to the guilt Kyungsoo developed from Dong-Hyuk being dragged into Yule's "cycle" and that played a part into thinking that he belonged with Yule because they were "perpetrators," then I gotcha.