From Horobi no Michi:

Kirisaki Fumio's just an ordinary highschool girl in love...

...but then she finds out that the guy she's fallen for happens to like someone else.

And then she gets into a fight with a friend... that and various other things are getting her really down...

...so she decides to go out and get a chainsaw.

And take it to school.

And kill the boy she likes, and herself afterwards.

Along with anyone who happens to get in the way.