This is because of Japanese military culture at the time. This culture was far from displaying mercy to anyone. Their are even similarities between the Japanese army and the Nazi's SS, who saw the ideal as a cult and that everyone who is weak must perish. The Japanese did not only limit their "no mercy" culture on enemy forces, but on civilians as well. They never understood the concept that there needs to be a balance between well treatment and what they did, because this will ultimately get more out of the prisoners.
