why just why, what’s the purpose of making these kinds of stories?

What kind of question is this? Not all writing or art needs a higher “purpose” or moral lesson, or to teach people healthy relationships lol. It’s fiction. The author likely wrote this because he/she found it cathartic to write dark things, or because she enjoys trainwreck stories where everyone suffers and everything goes wrong. Personally I enjoy reading these stories and can also separate fiction from reality.