After the romp, Kichizo went back to his wife and Sada grew depressed. When they met again, Sada acted out a scene from a play. Playing the part of a jealous lover, she threatened Kichizo with a knife. He was amused by this.
Later, when they were in bed together, she put the knife at the base of his penis, threatening to cut it if he cheated on her. He found this amusing, too. The pair enjoyed playing sexual games with asphyxiation-- taking turns choking each other during sex-- but when Kichizo fell asleep, she choked him to death, cut off his penis with the knife and wrapped it in a magazine paper to carry with her.
When she was caught, police reported that she became excited talking about the murder and that her eyes sparkled. She told police why she had committed the act: “I loved him so much, I wanted him all to myself. But since we were not husband and wife, as long as he lived, he could be embraced by other women. I knew that if I killed him no other woman could ever touch him again, so I killed him."