I think the parent is answering OP's question - which is about the _gunman_'s justification. The parent explaining behavior is not the same thing as the parent defending or justifying it.

Japan's stance of war crimes has been a point of tension between Japan and its East Asian neighbors.

These are matters of nationalism and affect forward looking geopolitical outcomes.

Following WWII, a tribunal was held in Tokyo along the lines of trying German war criminals. You can imagine how celebration of what others perceive to be German war criminals could lead to conflict.