Okay, here goes…
The rapists needed a base of operation; a place to avoid the authorities, where they can freely distribute their drugs to their consumers. Enter Mio.
I think the blonde boob was their first victim. She may have been the one who betrayed Mio i.e. releasing classified information about her to the rapists. In episode 1, when Mio was walking in the hallway, she notices the blonde boob exiting out of a classroom, with the same professor in this episode. Mio questions her wondering why she was with the professor, and she ends up making a “bs” excuse about going over the “funds” with him.
I think the blonde boob is the blue hair girl in episode 1. I mean, whatever happened to her, am I right?
In this episode, we know what exactly happened with the blonde boob. Prior to this event, she notices an embezzlement and confronts the professor about it… and the rest, y’know what happens. The “funds” were used for their drug-operation.
The maid was the real M.V.P of this series. She started putting a small quantity of drugs in their drink, so it goes unnoticed, and as the series progressed, the two rapists were over-dosing on their own recreational shit. She could have just dialed 911, imo… This was one of their two crucial mistakes. The other one was that they exposed their alibi, the professor.