
Wooyeon have ASPD, a mental disorder that we call nowadays as psychopath. He can't understand normal emotions and he also explains the unwritten intention to Inseop so that Inseop will not leave Wooyeon.
Unwritten Intention refers to an act that may cause harm to others (example: Murder, Rape, Arson etc.). He knows the consequences but a person with ASPD will still do it. (Remember the scenes in which Wooyeon did something to others knowing the consequences.)
Willful Negligence - negligent acts where the defendant disregards the risks of their actions and is aware (or should be aware) of the possible impacts.
I'm not defending Wooyeon for raping Inseop because it's not right. I'm just saying that Wooyeon has mental disorder ever since he was a kid and can't understand emotions like love, caring for someone, etc. There's a scene where Wooyeon will explain his mental disorder to Inseop (so that Inseop won't leave him. He did it for Inseop to pity him because a normal person reacts like that after he told his sad memories).
In the 2nd volume of the novel after the Hawaii part, he always suppressed his desire to have sex with Inseop (because Wooyeon wanted to take good care of Inseop's health after Inseop almost died) and obeyed everything Inseop said (If Inseop doesn't want something, Wooyeon will listen and not force his way). He's still a red flag but he always tries to avoid doing something bad so that Inseop will not leave him.
So for me, in the 2nd volume, he's a red flag but changed even though he couldn't understand normal emotions. He patiently endured everything for Inseop and always asked Inseop's permission before doing something.

No, he won't cheat on Inseop because Wooyeon is literally in love with him even though he can't explain what kind of feelings he has for Inseop.
Wooyeon doesn't want for Inseop to be kind to other people, not even Manager Cha and the CEO, Inseop's family, even the plants and animals. He just wanted Inseop for himself but he couldn't do that because he knows Inseop's personality, that he's just so kind. So he endured seeing Inseop being nice to other people.
He likes seeing Inseop cry because he's pretty but he doesn't want him to cry at the same time because he felt like going crazy, his heart aches if he sees Inseop cry. Also the moment he saw Inseop's smile, he thought to himself that he's pretty when he cries but he also looked pretty while smiling that he wanted for Inseop to always smile only for him, and not to other people.
He patiently waited for Inseop's consent before having sex because he didn't want for Inseop to leave him. He still keeps Inseop's passport btw.
He also can't sleep without Inseop. And ever since Inseop got stabbed because of Wooyeon's stalker, Wooyeon always worries for Inseop because he almost died. Just imagining that Inseop is not by his side, he feels like he'll go crazy and will be admitted to the mental hospital for the rest of his life.
It's the first time for Wooyeon to encounter an emotion that he didn't know. So the moment he realized that he can't live without Inseop, he wanted to be nice to him and avoided doing the things that can or might hurt Inseop.

Btw he realized that he loves Inseop in Hawaii part. Wooyeon thought that Inseop left him so he searched for Inseop for 3 hours straight then came back to he hotel they were staying. After coming back, he saw Inseop outside his door, holding a cake because that day was Wooyeon's birthday.
When Wooyeon saw Inseop outside, he felt mixed emotions. He forced Inseop to have sex with him, and confessed that he didn't know what to do when he couldn't find Inseop so he searched every airport (In the current manhwa chaps, Inseop will go back to US that's why Wooyeon was afraid that he'll go back to states and leave him).
Even though Inseop was hurting, he understood Wooyeon's feelings and allowed himself to love Wooyeon. Inseop confessed that he loves Wooyeon so he needn't to worry about him, leaving his side because Inseop chose to become his manager again and stayed in Korea with Wooyeon.
I love everything about this manhwa. The story development and the healthy relationship of everyone with each other! They experience tough times and made me cry the moment the brothers opened up with each other. I hope to see the story where Sung Woon and his Mom's relationship will be okay, as well as the side story of Hyesung and Taeyangg! Of course the side story of the Byeol ang Sung Won's future! Wahhhhh! I want moreeee!