Japan, like Sparta before it, believed in "Death before Honor", meaning fighting men would die before surrendering. Surrendering was dishonorable.
Allied fighting men surrendered routinely, it was part of war in their culture; there were even rules describing how to do it and how to treat men when they did surrender.
But again, like Sparta, it was NOT in "their" culture.
Consequently, when allied military men surrendered, they were treated accordingly; with disrespect. Disrespect in the eyes of Spartans or Japan translated to "war crimes" to the allies.
Translation: What was killing to one pair of eyes was murder to another pair of eyes.