It seems that Nadeko has gone somewhat crazy, because the person she likes got a girlfriend.

She eventually tries to kill Koyomi, but then his phone rings. It's Hitagi, who tells Nadeko to kill her instead, but the latter says she'll kill them all. So Hitagi tells her that the order is important, because if Koyomi dies, Shinobu will return to being the legendary kai-killer, so Nadeko will have to kill Hitagi, then Shinobu then finally Koyomi. Furthermore, if Nadeko's killing intent is certain ('eternal') rather than impulsive, Hitagi asks her to wait until after graduation.

The very last scene has Nadeko welcoming Koyomi, Hitagi and Shinobu at the (snake?) shrine in March.

"Koyomi-onichan, I'll give you all my affection, ok?"