Tune that Yamanaka whistles...

That is the song named "A young eagle's song" in the Japanese army, which was a popular among youth soldier's. I do not know if you know the history of Yamanaka Yamanaka, he was a survivor from youth soldier's special force, determining to death during the world war 2. Most of survivor felt guilty to be survive through his whole life, because so many his companies died.
Actually,the script writer, Mr. Kasahara was a survivor from that youth soldier special force. So, he wanted to put his-own feeling that was being shame to being survive in this cinema. Not only this topic, also please note the ending. The way of Yamanaka's suicide was the same as the soldier's suicide. Putting the barrel of the gun in to his mouth reflects that a soldier can not have a hand gun, but long rifle. Such long rifle did not allow him to put it into the his own head with pulling the trigger. So, Mr. Kasahara who was the script writer made Yamanaka did the same way of the youth soldier, even Yamanaka was able to put the pistol barrel on his own head, but he did so the way of the soldier.
This cinema reflects various kind of the feeling of Japanese after WW2.