How did the Japanese kill prisoners of war?

POWs may have been rewarded for duties performed, possibly even paid. But traditionally prisoners received little respect from Japanese military men, as surrendering was looked upon as a form of...

When the war ended.

one thing that they might have done for torture was cut off all their skin and kill them

Japanese POWs. There were only a very small number of them because they often commit suicide rather than surrender. Those who were captured however were treated according to the Geneva Convention.

Since this question is vague and confusing I will try to answer you. If you mean "when" did they start keeping prisoners of war that would be when they captured American forces and Filipino forces....