The manga opens with the shadowy image of a crying girl and a tale of a girl being sacrificed to protect the school. Now consider that we learn Yuuko was not only sealed in that underground space, but that she also had a broken leg. She died, scared, alone, pleading for help, and in pain.
Episode 4 of the anime gives us our first proper look at Shadow Yuuko as she stands over a sleeping Kirie, displaying a full on Slasher Smile full of dagger-like teeth.
Episode 6: Kirishima Yuuko being offered up as a sacrifice for the fictitious monster that she created as part of a Batman Gambit that had Gone Horribly Right. People she thought of as friends were willing to kill her to save their own skin, and the worst part is, there wasn't even any malice towards her when they were going to do it.
Episode 10. All of episode 10 is either this or Tear Jerker. First person Human Sacrifice is a scary thing. Summary: Teiichi is trapped inside Yuuko's body via memory flashback and forced to watch as Yuuko is thrown into a filthy pit and left there, as she slowly goes insane and dies of starvation, with her leg being broken and thus making her essentially immobile on top of it all. Let me repeat: TEIICHI IS HELPLESSLY FORCED TO WATCH YUUKO DIE IN THE MOST TRAUMATIC MANNER POSSIBLE. It's a wonder he was still sane afterwards...