What I read from previous spoilers Soohan wanted to KMS because he got SA-d that time by ...

☹noooooo

Does the ML know? Is that why he also hates the cousin? & trying to keep the MC away?

He didn’t know when he was a kid. He found out later.

By SA u mean he was raped or molested? Bc as far as I remember from the novel the Omega was not raped.

He was touched and that thing shoved into his mouth by his "ex-boyfriend". Well, they were not really dating, he was just a friend and his classmate. And MC felt somewhat obligated to do it cause that guy was financially helping him too. So I wonder if it could be considered SA.

Sorry, I was going to post this reply on a comment thread of another Manga but accidentally replied to you.

It was an attempted rape by the cousin. SH couldn't see him because he had an eye mask on but broke a lamp on him and cut his abdomen. SH believed for years that is was Yeonso who did it because he was the only alpha who could've been there at that day.

Where can I read the novel?

Novel updates or readhive..

Sorry but the TL is totally wrong..
SH tried to KMS and strangle YS when he was in middle school.. he wasn't in the best place mentally, you'll understand once SH's past before he came to live with YS and his family is revealed.. SH was suffering..
After that he got into high school and went to the dorm.. later he went to study in France for college..
the SA happened when he came to visit while in college bc YS was about to enroll in college in US.. when SH was sleeping with his sleeping mask on he was in heat and YS's cousin who was in the house looked down on SH whom his parents praised over him.. so the cousin decided he would "teach a lesson" to SH.. he tried to force himself onto SH but SH smashed a lamp and stabbed the guy with a piece of glass.. he did not reported the incident bc he was worried he might get sued for excessive self defense.. however since SH was about to meet the family in their beach house he thought YS was the one who assaulted him..
SH knew YS liked him and he also knew the parents kept him around as a "future bride" after he turned out to be an omega.. he thought the parents covered it up and acted like nothing happened to protect SH.. so he was resentful of YS.. the accident happened when he was about to go get evidence about who assaulted him..
SH is actually scared of YS who has everything loving him who has nothing in a way..
SH has quite a lot of issues and this story is his journey of overcoming them..

When does SH know the truth that it wasn't YS who assaulted him?

Before the accident, he didn't "know" it. He initially didn't give much thought to the incident. But then he started suspecting it, he didn't want to believe it wasn't YS bc that would mean he didn't have a valid reason to not accept YS. So to prove himself right he went to get the evidence and otw got into an accident..
After the accident, SH has not pre-concieved notions about anyone or anything. When SH gets kind of a flashback/dream about the incident after going into the same room he was assaulted in, he starts comtemplating who it was. He questions why would he not recognize YS's pheromones if they lived together after getting married but he realizes YS takes suppressants to not have his pheromones leak. But he ends up ruling out YS after probing him a bit and realizing his body does not react badly to YS's pheromones. He then starts suspecting the cousin bc he feels averse when he comes near him. But he is not sure if its bc its a trauma response or simply bc he does not like the cousin. He kind of becomes sure when he realizes just like he himself started taking suppressants to make sure his pheromones never leak out bc he was traumatized by the incident soon after it happened, the cousin too started taking suppressants to the point none of his pheromones leak out. He also remebers the first time they met at the company after he joined and the cousin was tense while asking "have we met before?" And he relaxed when SH replied that it was their first meeting..