The Hunter Becomes the Prey

Kai Munakata is a slightly shy(?) mask-wearing boy who's in love with his senior, Aizawa.

He does things like gathering secret audio recordings of Aizawa-senpai having sex with other boys and collecting used plastic bottles she drank from... and due to the nature of his warped love, he keeps repeating these acts of stalking.

One day, while he's secretly listening on the bus to his audio bug recordings of her, Aizawa-senpai herself goes to sit right beside him...!?

(Source: DLsite garumani, translated)