IMO, as for why Japan was so cruel.
A poisonous mix of :
1. A sense of superiority over their neighbours
2. Bushido (Way of the Warrior) was promoted extensively in the military
- to be captured and not die bravely in battle was worse than death. People who willingly allowed themselves to be captured were seen as worse than offal.
3. An education, pre-military and in military, that promoted 1 and 2 and the view of the Emperor as God-like and infallible.
Just to remind people. Until the Emperor made his announcement on the radio that japan was surrendering to the Americans.....most Japanese thought they were winning the war.
How could that possibly be? Well for a modern example you just have to travel 400KM west of Japan to see a perfect example in North Korea.