It's not complicated guys. The mother is abusive and the negative feelings piled up in the daughter until she finally snapped and killed the mother.
The author didn't make it subtle at all. The daughter is less than 17 years old (since she said she would die at 17, so she must be younger than that) but her room is filled with dozens of trophies from every extracurricular activity imaginable. Clearly, her mother was putting a lot of pressure on her to perform, without ever allowing her even a single day off (as seen when she asks for a single day off and her mother agrees only to renege when the day actually arrives), and using her as fodder for her Insta (as seen when she takes 5 selfies with the daughter in one chapter). The daughter finally acts out and "runs away from home" for 2 days (to her grandfather's house), meanwhile, her mother is out on the streets taking selfies because she doesn't actually care. Then the daughter gets prompted by a friend and asks not to move away, so the mother makes a big show of being understanding and agrees.
Then the daughter's friendship hairband "mysteriously disappears" and her mother offers to replace it with a family heirloom... but then the daughter finds out that the "family heirloom" is some random junk that sells for 2 bucks on the street.
It's clearly an abuser-abusee relationship.