What is the importance of Mr. Igasaki's house in the novel, Earthlings?

Mr. Igasaki's house is in Mirai New Town near Natsuki's house. He lives there with his parents since he is a college student. When his parents go out of town for the summer, he tricks Natsuki into coming over so he can force her to perform oral sex on him. He tells her she must come back for more "special lessons" with him throughout the summer, and shows her where he keeps the spare key. Instead of doing as he demands, Natsuki sneaks over to his house in the middle of the night with a garden scythe and uses the key to sneak into his bedroom and murder him.