Did you forget?? He was supposed to be an alpha (his exams said so) but he was exposed to a dominant alpha's pheromones for a long time (aka Su-ah's pheromones) and his sub-gender changed, same thing with his heat, bc Su-ah kept showering in pheromones he became irregular (I forgot if this is the exact word they use), they lit showed the MC punching him at school bc of this.
I checked the manga again, but couldn't find anything that said it was the ML's fault that the MC turned into an omega. It only states that the excessive coding in alpha pheromones made controlling his omega pheromones harder and heats irregular. Check chapter 2.
Maybe you got it confused with another manga? But could be that i missed something, or that part of the manga wasn't completely uploaded here, noticed when I was going through chapter 1, it seemed like a part of the manga was cut out/missing.
That's true (check chapter 2), even if his family is full of dominant alphas, there can always be an exception (MC in this case). I do know that excessive alpha pheromone exposure can turn someone into an omega, but that is very, very rare. The ML was still a child, and to my knowledge, people develop alpha and omega traits at a certain age, which is the age they get their test done to see what their secondary gender is.
Turning a person into an omega takes years and requires constant exposure to the same alpha's pheromones.
So, logically thinking, it wouldn't have been possible for the ML to change the MC's secondary gender; hence, they just developed their traits.
Yeah as far as I remember there's no part that explicitly say that Juhyeok being an omega is Su-ah’s fault. Their relationship indeed got worse since the trait’s been decided but that’s it, moreover that is because Juhyeok wanted to be Su-ah’s alpha at first so his ego and expectation got wrecked because of it. (His resentment towards alphas get worse as time goes by because of his mistreatment)
Chapter 1: he found out he is an omega and he gets tested, his mom literally screams that he was pe-escribed as an alpha. Yes, you said that he was exposed to his family's pheromones, but not only (as long as I know) that doesn't affect family (I mean pheromones), bur Su-Ah was already an alpha (a dominant one).
Chapter 4: lit had a fight at the beginning where they explain that bc Su-Ah showered him in pheromones the MC has a hard time controlling his own pheromones (making him irregular)
Chapter 14: The mc screams that Su-Ah made him so he couldn't live w/o him (it can be bc of the cum stuff, but the cum stuff only works bc Su-Ah's pheromones where the ones making it hard for him to control himself to begin with)
Chapter 1: he found out he is an omega and he gets tested, his mom literally screams that he was pe-escribed as an alpha. Yes, you said that he was exposed to his family's pheromones, but not only (as long as I know) that doesn't affect family (I mean pheromones), bur Su-Ah was already an alpha (a dominant one).
Chapter 4: lit had a fight at the beginning where they explain that bc Su-Ah showered him in pheromones the MC has a hard time controlling his own pheromones (making him irregular)
Chapter 14: The mc screams that Su-Ah made him so he couldn't live w/o him (it can be bc of the cum stuff, but the cum stuff only works bc Su-Ah's pheromones where the ones making it hard for him to control himself to begin with)
Also, they already said that Su-Ah used to shower him in pheromones whenever he got jealous (and we can also see it in MANY chapters), literally the first time they encountered the sugar daddy was bc Su-Ah let out his pheromones bc he was afraid something might happen to MC (and we can aslo see how is not the first time that happens bc in later chapters when they are in the same room as the sugar daddy, Su-Ah lets out his pheromones to....resort dominance?? I don't remember the word, sorry)
Sigh. Juhyeok found out he’s an omega BEFORE Su-ah abuses his pheromones. He was prescribed as such because his family consist of dominant alpha, even he thinks that way. But it doesn’t mean he is sure to be one, he's one of those rare cases where a family that gave birth to an alpha for generation produced an omega, even his mom was shocked. So it’s not because of Su-ah.
"DIDN'T YOU SAY HE WAS AN ALPHA IN HIS PRE-SCREENING?!" Literal text from the manhwa, pre-screening, as in, they tested him and he was supposed to be an alpha. His family doesn't consist of dominant alphas, just alphas for generations, that's why she is confused, and they also said that it was one of those rare cases where, bc of pheromones exposure, the gender changes. We are reading the same story bc, as I remember, there's literally a WHOLE chapter showing how Su-Ah discovered he was an alpha and then tried to make himself look smaller and cuter as he spent time with the MC bc he (still) thought the mc was going to be an alpha, and AFTER he discovered the MC was an omega, he went to the mc and confessed, bc he wanted to be the mc's alpha . They don't have to literally write everything word for word, they make context clues for a reason, and a timeline for the same reason

So this right here is also another reason why the MC is mad at Su-ah (he often uses his pheromones on him, and also his pheromones are the reason of him being an omega)