I'm pretty sure the plan was to kill me and pin it on Kirigiri-san. When she couldn't do that, she decided to put Ikusaba-san's body in the garden.

[ He pauses, thinking back to the time he spent among piles of garbage in that wretched room. He still remembers "Detention" and what had followed afterwards with too much clarity for comfort, but he...is okay with talking about it. He can do this. ]

The second time hasn't happened for me yet, but it was after my failed execution. Alter Ego saved me by making me fall into a garbage room but the door was locked and I couldn't find any food or water. Without Kirigiri-san, I'd die of dehydration.