Sounds like a domestic death cult. Like the one that tried to release nerve gas in a subway there twenty years ago.
I’ve been to Japan a number of times; a remarkably safe, clean and orderly place with basically no street crime and very gracious, pleasant people. But a friend who married a lady from Japan told me once when I asked him about the Tokyo subway poisonings, “when the Japanese go bonkers, which is very rare in their society, they really flip out”. I suspect that happened in this instance.