The MC was abused by his mom, and similarly his mom was mistreated- maybe she was unstable to begin with- but she was further pushed to the edge to the point she wanted to disappear even early in his childhood- by the relatives.
I'm not defending the mother's actions, but rather saying- as with most things and most people- there's a context. She was made the way she was just like you're made the way you are.
As someone who can sympathize a lot with the story- no tossing kids off cliffs involved nor tearing up love letters- but plenty of corporal punishment, moving countless times, isolating from extended family and later after shit went down- a fair bit of undeniable abuse (both my parents were fucked up, then, quite frankly)- that's how it is.
I appreciate the author making a more complex character- not a villain so much as just a fucked up person as all people are (I'll just go and say I agree with the whole "humans are the real monsters" trope, not all are, but to not be one is something that needs to be built, learnt, given the right environment).
Can you feel sympathy for the mom and then also denounce what she did? Yeah. Can you feel sympathy for people and hate them- even knowing their buttons due to the power of empathy- use them to destroy them? Also, yes. And so on. I wouldn't say the mom is necessarily "irredeemable," but forgiveness is just that- sentiment, and not something owed or whatnot. Some things, most things can't be undone. You just have to live with it and move on.
In a sense I'd bring up Hitler as an example of sympathy, though- if you want to look his history up. Grew up possibly an illegitimate child (ironically possibly part Jewish as a result), had an abusive asshole of a father whose Austrian nationalism probably contributed more than a bit to Hitler's pan-Germanism (as a middle finger to his father's beliefs), experienced homelessness in Vienna and failed to get into art school, and went through the traumas of the first world war and experienced mustard gas-
He's a monster, but the worst monsters are always human. I don't say that to claim some "let's treat everybody kindly," "both sides," or whatever bullshit pacifistic wishy-washy nonsense, but rather because when you acknowledge that you're no longer just dealing with the idea of them, but them themselves. Empathy allows for better cooperation- better animosity- even, when deserved, better cruelty and retribution.