Well things are already bad as it is so the plot twist didn't affect me at all, naturally Eyama would feel nothing too. The guy just wants to end it all as quickly as possible so stfu Niimura.
"I arbitrarily decided to rape your girlfriend because some other guy raped some other girl, who just so happens to have been your girlfriend's sister. You should take revenge on that other guy, not me!"
Didn't Arakawa literally torture and murder her? Somehow raping her on top of that is what haunts him the most. Or is it that he regrets getting the sister involved as well? In either case, it sure does seem to be a flip flop from the previous chapters. I don't know why I'm trying to make sense of this story or expect anything from these characters though.