That depends.
The omega "snuck into his house and had sex with him during his rut and bonded with him". Either the omega manifested during DoYoon's rut inadvertently or he had done it intentionally. Idk. The beginning of their relationship seems very loving. We don't know what happened throughout their sexual relationship though. Maybe their relationship wasn't clear or the omega has unrequited love. I don't understand YeJun's motivation for running away after the rutting sex, but he did mate and bonded with DoYoon against DoYoon's knowledge and will, so he's not a good person. It could have been unintentional such as a manifestation of omegahood during the rut, idk. But DoYoon had NO KNOWLEDGE of whom he had been bonded with. He also prob broke up w/ YeJun prob because he had been bonded. That and or their relationship truly was just FWB. But unless DoYoon is a sadist, why would he remain w/ YeJun and have a hard time having sex w/ a beta when he CLEARLY could have had much more enjoyable sex w/ an omega?!? On top of that, he is now bound to some unknown omega, so he can't bind to anyone else, can't have kids with anyone else either. YeJun is not innocent here. Unless we see more of their past relationship and understand more of YeJun's motivations I don't understand why you'd hate the alpha so much.
Just once I would like the main character to fall in love with the friend who treats him nice hand is caring caring who is loving instead of the ruthless POS Alpha who hurt him