In "Patriotism" by Mishima, why is Shinji determined to die by ritual suicide?

In "Patriotism," Shinji feels that he must die by ritual suicide because he feels he has no other choice.

Shinji feels that suicide is the only honorable path to pursue. Members of Shinji's unit, the Konoe Transport Battalion, had sided with "the mutineers" against the government.  These battalion members participated in attacks aimed at overthrowing the prime minister and other government officials. The uprising failed, and the mutineers had been captured.  When he returns home, Shinji tells his wife the pain he feels over his predicament. He tells Reiko that he will have to carry out the "Imperial ordinance" the following morning.  Since members of his unit will be "posted as rebels," he will have to attack them.  

Shinji feels that he cannot carry out this order and that "it's impossible to do a thing like that."  After a pause, he repeats his agony to his wife:  “They’ve taken me off guard duty, and I have permission to return home for one night. Tomorrow morning, without question, I must leave to join the attack. I can’t do it, Reiko.”  When he tells her that he cannot attack his comrades, she understands that he feels that he has to kill himself.  He is "resolved" that suicide is the only choice he can make.  He chooses ritual suicide as a way to find honor in a situation that presents only dishonorable options for him.