Theory: Nekozaki liked someone before, and her feelings for this person hurt them/herself, probably leading to a situation where it could be interpreted (by a guilt-stricken Nekozaki) that her feelings had killed this person (who may or may not be actually dead, but certainly their life situation took a downturn).
For Arata Iri that's rather dark, which piques my interest. A slow (or sudden) slide from light-hearted slapstick comedy into scandal (teacher/student relationship) and drama (overcoming Nekozaki's past) sounds like a great read.
While I don't subscribe to the "Nekozaki is the actress student" theory (I'm convinced the shivering in that scene is the prelude to her blowing up at the other girls in the next panel), it would be interesting if that were so.