The Japanese were a warrior culture who believed that to surrender was tantamount to admitting that you were a coward. The European soldiers, including the US Army believed in the idea you fight until you know you can't win and then you surrender in good faith.
This lead to the idea that they could treat the cowards in their care any way that they felt they should up to and including killing them they were able to slap prisoners and work them to death if the wanted to, and they did.
Another thing to remember is that the Japanese considered it cowardly to allow oneself to be captured. Honor meant everything to the Japanese soldier, who would sooner die than be taken prisoner.
So, the combination of a superiority complex wedded to a somewhat misinterpreted code of honor that held that the greatest honor was to die in service to the emperor played a part, I'm sure, in the mistreatment of civilians in occupied countries as well as prisoners of war (and even those unfortunate foreign nationals who were interned).
The Japanese believed that you should not surrender to your enemies and should die in the attempt to win the battle/war. They did not believe in the Geneva Convention which is there to assist prisoners of war and civilians, so they could be as cruel as they wanted to be. It is just their type of culture; thankfully not everyone is the same
The answer lies, at least in part, in the fact that the Japanese considered themselves to be superior to other Asians, and an almost fanatical devotion to a somewhat corrupted version of the samurai code.
This code prescribed suicide rather than surrender
So, the combination of a superiority complex wedded to a somewhat misinterpreted code of honor that held that the greatest honor was to die in service to the emperor played a part, I'm sure, in the mistreatment of civilians in occupied countries as well as prisoners of war (and even those unfortunate