Ah, interesting little side stories. This ship is pretty awks cus we've hardly seen of them, as well as the fear for Shiena's life when she's with Otoya, but it's nice to that her killer urges are shown to be a little bit more toned down when it involved Shiena. It makes the ship more comfortable for me, when I don't have to worry (or at least, worry less) about one person raping and murdering the other..
On a side note, "advenge" isn't a word. I'm not sure I get it either. If Shiena was to "avenge" this girl (therefore, get revenge on her behalf, since the girl helped Shiena with her own revenge it seems), and Otoya kills that girl, how does it lead to Shiena owing Otoya a favour? I'm not sure why killing the person one owes a favour to, leads to owing a favour to the murderer. It seems more to me that Shiena has to avenge that girl's death instead, since she got fucking killed and all. Idk what the girl wanted Shiena to initially do, but surely, whatever wrong was done to her, isn't greater than getting murdered