The whole archipelago will turn irretreivably xenophobic in a New York minute, the drawbridges of the Japanese mass psyche will go up immediately.
If it is North Koreans who do the shooting and bombing and gassing, you can expect a huge, violent backlash against ALL Northeast Asian foreigners in Japan: Chinese, Korean (both South and North), Taiwan, Mongolia.
The sleeping giant of paranoid xenophobia will have been fastidiously awakened. I wish the Japanese law enforcement and domestic intelligence authorities could anticipate this...but alas, I wish for a lot of things that don't come true.......