The Japanese Constitution was created by the United States which occupied Japan after the Pacific War. The big feature is abandonment of war. The United States that received great damage in the battle with the Japanese army wanted that Japan had no great military force after the war. Therefore Japan can not invade other countries with force. However, Japan has the Self Defense Force (SDF) to protect the country from attacking by another country.
By the way, SDF of Japan is also required to cooperate for PKO in overseas conflict areas in recent years. However, their activities are restricted to digging wells for local residents, or supplementing supplies to allies, due to constraints of the Constitution.
Therefore, the SDF is not killing any persons in the battlefield as anyone until today.