[4][5] A photograph of the Asanuma assassination taken by Japanese photojournalist Yasushi Nagao won World Press Photo of the Year for 1960 and the 1961 Pulitzer Prize.

He was the second son of Shinpei Yamaguchi, who by 1960 would become a high-ranking officer in the Japan Ground Self-Defense Force, and was the maternal grandson of the famous writer Namiroku Murakami, well known for his violent novels glorifying the chivalric code of Japanese organized crime syndicates known as the yakuza.

[8] On 10 May 1959, at age 16, he heard a speech by right-wing ultranationalist Bin Akao declaring that Japan was on the verge of a revolution, and that the youth must begin resisting the actions of left-wing groups.

Later in the interview, he stated that he had resigned from Akao's party in order to "lay [his] hands on a weapon" and be free to take more "decisive action.

At the time of his arrest at the scene of the murder, Yamaguchi had a notebook in his pocket describing Asanuma's actions as unforgivable, as well as detailing his motivations for the attack.

In this note, he made mention of political controversy surrounding Asanuma's support for the then recently proclaimed People's Republic of China, the June 15th Incident during the Anpo Protests, as well as concerns over Japan potentially becoming communist.

If even a single member of the general public that is now blindly following the blandishments of left-wing agitators were awakened to their folly, I thought it would be worth doing...[11][b] Yamaguchi spoke of Akao respectfully, referring to him using the honorific sensei (先生, master), however, also stating that Akao was more interested in "media attention" and "agitation", as opposed to actively putting his words and ideals to practice.

[3][17] Right-wing groups celebrated Yamaguchi as a martyr; they gave a burial coat, kimono, and belt to his parents and performed a memorial service for him.

[19] A photograph taken by Yasushi Nagao immediately after Yamaguchi withdrew his sword from Asanuma won the 1961 Pulitzer Prize,[20] and the 1960 World Press Photo award.

[18] Yamaguchi's actions and the massive publicity they received inspired a rash of copycat crimes, as a number of political figures became targets of assassination plots and attempts over the next few years.

In this incident, Kazutaka Komori, a 17-year-old member of the Greater Japan Patriotic Party, attempted to assassinate the president of Chūō Kōron magazine for publishing a graphic dream sequence depicting the beheading of the emperor and his family.