The incident happened after the perfect crime... The conspirators are disappearing! Who is behind it?

Iwao, , Kumagai, and Konishi broke into the safe containing the wages of the employees of Ato Industries and obtained a fortune large enough to live a life of leisure. They celebrated with Iwao’s woman, Rumiko. However, it was too early to celebrate. They decided not to use the money for six months to avoid suspicion and stored it in a safe. But one day, Konishi asked for his share of the money in advance, even if it meant receiving a smaller portion, igniting the dark flame of desire among Iwao and the others.