It might help if you tell us what you consider a simple art style. Do you mean a stereotypical shoujo art style, with large eyes and sparkles? Or do you mean something like Natsume Ono's "Not Simple", which some would consider different but also simple? Also, are you looking for a manga or an anime?
Female killers are pretty common, but the only ones I can think of where a small town might pop out in memories is the Higurashi When They Cry series. I haven't read it in a long time though, so I don't know if there's any parent murdering going on. Probably some.
If it's a manga, I think checking the tags at mangaupdate ( https://www.mangaupdates.com/categories.html) might be the best option for you, since female killers are actually pretty common. With that description it might be best to go to the serial killer or mass murder tag and start clicking until you find something that looks like it.
trying to find this one horror thing
the art was... different and i think simple
i dont remember much so im sorry anything of the sort is welcomed
but its was in this small town and this girl was killing other people and students and the girl was killing the other students and parents and stuff