Sugimoto had failed the police entrance exams three times. At this point he was already on the brink of giving up, but then a certain politician had approached him and aided him in getting into the police force. In exchange, Sugimoto would have to follow his orders.
It is also of note that Sugimoto had been in a relationship with Yamagiwa.
Sugimoto had been contacted by the politician which he had been working for to get rid of the evidence Yamagiwa had against him. Sugimoto had called Yamagiwa out early in the morning to talk about it.
Yamagiwa had refused to hand over the evidence and so Sugimoto had threatened her with a gun. When Yamagiwa had known Sugimoto could not shoot her, he changed his threat to shooting himself in the head.
Yamagiwa had panicked at this and tried to stop Sugimoto. In the process, he had accidentally killed Yamagiwa by shooting her in the chest.
The diet member had then told Sugimoto how to cover up his crime.