[FlameOn]

Aside from the discussion with the suicides, Japanese were really cruel because of the culture of Bushido surviving in the military following the collapse of the Tokugawa shogunate following the Meiji Restoration. Surrender or even capture was the highest level of shame that a person could commit. If you surrendered, you'd probably be considered worse than a dog... so... the Japanese felt they really could treat you however they liked at that point. No need to waste resources or effort humanely treating POWs and captured civilians.

This attitude prevailed throughout Japanese military elite at the time and under pressure from them, the Emporer signed an order in 1937 allowing the military to not be constrained by international law regarding the treatment of prisoners. This snowballed to the Japanese having standing orders to execute prisoners at sea by 1943.

The above, coupled with general Japanese xenophobia and outright sense of superiority at the time and the rest is history, 30 million dead at the hands of the Japanese.