Tengoku is the type of story that can only be done in a manga, which is one of the reasons I appreciate it so much. You can't really leave the kind of visual clues and other details in a purely text narrative.
I'm wondering if Robin was actually resentful and jealous of Haruki for having an alive sister to be with, which can explain some of his behavior, both with murdering Kiriko, and the sexual assault.
If Robin is still hearing "Marin"s voice inside his head (that crying scene felt like Robin was actually talking TO Marin, saying "Sorry you died because of me...", with her possibly responding in his head), perhaps he thinks he's "combined" with Marin in a lesser way than Haruki combined with Kiriko, which could have brought even more resentment when they reunited, but also to try and figure out how to recreate certain procedures to put the "Marin inside his head" into a real body again, using Hiruko combinations and brain transplants. Perhaps him growing out his hair out and resembling Marin so much wasn't just being unkempt, but some initial attempt at this... transformation.