When Sangwoo found out she had a lover, he said he waited to go to school with a fever because he was afraid his father would have beaten her. So he knew his father beat her up ( she's also shown covered in bruises in a scene with child sangwoo, while the scene with the lover Sangwoo is 12 )
The father was abusive before and you can't rule out he was abusing her even before Sangwoo was born. Maybe she didn't want the child, maybe she was forced in that marriage. I don't think Koogi would create a character evil for the sake of it. Women can abuse, manipulate and rape as well and it's important to show it, but she was really a victim and maybe she too has her reasons to be like that.
And she is not the only one to blame, because Sangwoo is also physically violent like his father to weaker persons like Bum, he did to Bum the same things his father did to his mother ( he forced Bum to eat at the small table just like his dad did to his mother) and now he's afraid to die like him, by the hands of the person who should love him.
There isn't anything indicating he abused her before that scene, that's just speculation on your part. What has been shown is that Sangwoo's dad was normal at first and trying to live happily with her and Sangwoo while keeping her from being crazy, then time passed and you can see he became and alcoholic and started to abuse her. That is the story shown, none of that speculation that she was abused before he was born or didn't want Sangwoo. She's just crazy. Whether he was abusive before the lover or not is only slightly relevant because what that scene shows is another instance of her real personality. She acts like a victim, but in actuality she's cunning. She let Sangwoo see her having sex with that guy and told the guy not to worry about being caught because "he pities mommy"... Besides, how is cheating a normal response to domestic abuse? Wouldn't she be scared to piss him off? No, she's being manipulative. Why'd she kill him with poison instead of stabbing him in his sleep? Why is she now trying to kill Sangwoo for seemingly no reason while acting motherly? She isn't a victim, she's a crazy bitch with a victim card to hide behind.
She doesn't let Sangwoo see her having sex with the guy, because she didn't even know Sangwoo was at home . She knows she can use the victim card with sangwoo, because Sangwoo knows his father beats her up, and his father really beats her up. She has bruises in several scenes, it's not that she invents that, he actually abused her!
I don't excuse the cheating and the manipulation over his child, but this doesn't mean she wasn't a victim of her husband.
And she used the poison for a simple reason, she was trying to get her husband weaker, because a weaker person can't physically hurt you. The same is doing with Sangwoo, he's a big guy and he looks like his father, so she poisons him little by little so she can have control over him.
She is not a criminal mastermind who wants to see the world burn. She is delusional and she maybe still dreams the day when she was happy, if she ever was. I'm not excusing her again, but it's clear she needed help and pity as well but nobody really helped her.
"she didn't let Sangwoo see her having sex with that guy"- You like winning the little battles, don't you? What i meant was that she knew he saw and didn't go to console him or worry about being caught. She told the guy not to worry because she knows that he feels bad for her, it's manipulative. She just let him feel like shit and didn't explain anything because she knew he wouldn't say anything. If you're looking at the scene, you know what i meant, picking apart my wording is getting no one anywhere.
"She is not some criminal mastermind"- I guess we'll see about that. I think it's pretty explicitly shown that she's just acting like a weak, innocent victim, especially within the last chapter. That... Was the reveal... There isn't really anything implying she was trying to "weaken" anybody, she was pretty carefree while she was making Sangwoo think it was his fault his dad died and when she saw him strung up about to be buried. It's in these instances where you get a glimpse of how she really is. If you want to believe she's poisoning Sangwoo to feel needed, fine, it's something that i could see being the case, but i'm going to point out that there is nothing implying that. Right now we're supposed to assume she is just crazy and is trying to kill Sangwoo.
I see some, for lack of a better term, SJW's talking about "victim blaming" Sangwoo's mother and it really seems like they're taking her side because she's a woman. Did we read the same manhwa? You people keep pointing out that she was abused like we didn't JUST get a reveal that she was actually fucking nuts even before he started abusing her. It's actually implied that he turned abusive BECAUSE things weren't going well between them, but it showed that he was trying to live happily with them, remember? She went to smother child Sangwoo, he saw, freaked out and stopped her before saying "we said we were going to be happy together!". Then it showed him fatter and abusing her later. It's implied he was trying to make things work, but she drove him nuts, he turned to alcohol (fat belly) and THEN he turned abusive. She just said he was "so cute when he proposed to her", this implies HE loved HER as HE proposed to HER. The fact that she was sleeping with another man around that time (BEFORE he started to abuse her as, if you noticed, Sangwoo was still a child in that scene and dad isn't abusive till he gets older) implies she might've been playing around with Sangwoo's dad. Notice how good her acting skills were as she played the mother role while poisoning Sangwoo this chapter? I'm willing to bet that mom had been using those pills in dad's food for a bit too. He probably realized how she is (cheating on him, trying to kill their child and now we know she likes to poison food, she was their cook.) and over time lost his shit. What's worse is that these people are completely denying mom's direct connection to Sangwoo's current state "because she's a victim". She is LITERALLY the reason he is the way he is, you can see it in the way he never trusts women and views them as dirty things, in the way he really only kills women, in the way he dressed Bum as his mother and in the way he had to have Bum prove his love without a doubt before he started to open to him. Even THEN he only JUST separated Bum from the image of his mom in his head, that's what the 1st bj scene a few chapters back was about. It was him accepting Bum as Bum, not his mother. A 2nd, separate special person to him. Sangwoo said it himself, his father scared him, but it was his mother's tears and screams that really effected him. He wasn't okay with his dad's abuse, but it didn't matter enough to him to become a killer, what really effected him was his mom and she fucked him up.