My reply contains spoilers, read at you own risk:
Even as a child, Yool's behavior was manipulative and sociopathic, and he carried these patterns into adulthood. Abuse may explain his development, but it does not excuse his continuous manipulation. You'll understand this more fully once you see what he does later (or read the novel)
Plenty of sociopaths come from abusive backgrounds, but abuse is not an absolution, it's context. The reason Howoon doesn't feel resentment isn't because Yool treated him kindly, but because Howoon has an avoidant personality shaped by trauma. He avoids pain and clings to the pleasure of the moment, not because Yool spared him harm. This is showed in the webtoon but the narrative is even more clear in the novel.
Yool is a bad person, deeply manipulative, fully aware of his actions even as a child. He never regrets his choices as an adult. If he hadn't believed Howoon recovering his memory was inevitable, he would have gladly kept him ignorant. He was always prepared to hold onto Howoon regardless of what Howoon wanted. The only restraint was his promise to Howoon's father and even then, he found a way to breach that promise without technically breaking it. Yool IS outright evil and destructive, as he is willing to kill everyone if Howoon refuses him. He feels absolutely nothing when it comes to killing others, even innocent ones. He is the grand archetype of a villain, which is what makes this story interesting as it gets revealed.
it is very normal to resent the person that literally killed your mom even if it was an accident when they were nine lol. people resent other people all the time for much less. mc is normal, it makes perfect sense with his character.
i get the comments wanting him to resent the ML to some extent but I don’t really see it being in alignment with his character if he does hate him, Yool was 9 and barely had any control of his powers, on top of his parents treating him like a monster, he didn’t have any choice or control over how he went bezerk w his powers. And it MC did resent him he would just be resenting a 9 year old, even the MC erasing him memory was selfishly done by a 9 year old. I get that Yool is still not best person now but he isn’t outright evil and hasn’t treated MC badly so the resentment would be unnatural to the character /: