Saitama Chainsaw Shoujo

I didn't get the flow of the story of this manga. From what I know, she had a delusion of rampaging in her school and killing everyone by using a chainsaw for the sake of finding her beloved Takumi, the one that broke her heart, and killing him then herself after that. Then in the end, they talked about alien enemy or something. That thing really confused me. I don't know if either she came back from her consciousness in reality or she was still in a delusional state after killing her clone. And lastly, she jumped off in a hole that was made during the battle then that ends the manga. I am not sure if she really did fall or if the story is centered on a delusional mind of the protagonist.

She did actually kill everyone in the school. That wasn't a delusion. They all just happened to be replaced with alien-look-alikes so the human students were just fine. The aliens took the boyfriend away because they need him to fight their own war for whatever reason (which doesn't really make any sense either way but it would help if you had seen something like Black Heaven). The broken window was apparently a path used by the alien girl to escape with the boyfriend.

tl;dr: This manga is all sorts of crazy and was really only written to have a girl do some chainsaw-fu. Don't think about it too hard.