So it is revenge because he got charged with sexual assault/molestation by the JK that leads to him being fired and divorced, at least the plot is believable this time
the JK scammed the guy around half a year before this happened. the guy got accused with chikan / sexual harrasment. he handled it wrong and somehow got himself admitting the accusation. he lost his wife, his kids, his jobs, and forced to pay several hundred thousand yen fine.
half a year later he found her baiting people for compensated dating in SNS, completely oblivious against the guy he scammed half a year ago. after the classic plot of stealing their wallet while the guy is in the bathroom, the guy drugged the girl, raped and blackmailed her, then fed her 'after-pill'. later in the story it is revealed that what he fed her was fertility pills.
8/10 great art, great plot, but the ending is kinda abrupt.