Mesu Kyoushi 3 ~In'etsu no Gakusha~

The protagonist, who has amnesia, does not know his identity, but he is able to live comfortably with the aid of his homeroom teacher Touko, and the rest of the school.
However, due to his lack of memories, he lacks dreams and hope, and feels an emptiness that makes him unable to grow attached to anything.
Then by chance, he learns what happened in the school and decides to explore the incident that cost him his memory.

He sneaks into the staff room but is caught by Touko. To her, he reveals his emptiness over being an amnesiac.
However, Touko is angry and rants over the pain of being a teacher for an amnesiac.
When the protagonist finds out about her true feelings, he is driven by a powerful lust from what he hears, and then he rapes her.
The taste of pleasure and excitement since he lost his memories, and the joy of screwing a woman is what he's been missing all along.
Then, following his awakened desire, he decides to rape the other attractive female teachers just as mercilessly.