What motivated the Japanese to be so vicious and cruel to their prisoners during World War II?

I am currently doing a PhD in history. My research area is war crimes trials for Japanese POW camps, so this is right up my alley. Part of my research involves reading a whole lot of war crimes trial transcripts, so that's pretty neat.

It is undeniable that during the Second World War, large numbers of Allied prisoners died in Japanese military captivity. Estimates from the International Military Tribunal for the Far East (IMTFE), more commonly known as the Tokyo Trial, place the number of deaths at approximately 35,700, out of approx. 132,000 (27%). This number, however, does not accurately reflect the total number of prisoners taken, mostly during the first 6 months of 1942, as it only counts white soldiers, and some Indian soldiers. A further 200,000 or so colonial troops and civilians were also captured. The common perception of Japanese treatment of captives is, as you have pointed out, vicious and cruel. This perception, however, has begun to be questioned in more recent academic works.

A significant example is in Gavan Daws', Prisoners of the Japanese, which shows of the 35700 Allied deaths mentioned at the IMTFE, approximately one third died at sea, mostly in submarine attacks. While there have been claims that the Japanese navy ought to have properly marked POW transport ships, to avoid friendly fire, no Allied or Axis nation did so during the Second World War. Gregory Mincho argues that thanks to intelligence interccepts, Allied commanders knew which boats were carrying POWs, but did not inform submarine commanders, in the fear that they would not be as aggressive in sinking the Japanese merchant fleet that transported POWs, oil, weapons, soldiers and associated resources. Therefore, over one third of the deaths can be attributed to accidental friendly fire and collateral damage, rather than Japanese brutality. Though I don't think arguing by numebrs is always helpful in such situations, this makes the death count more like 25,000 or 18%, still considerably higher than Western Allied POWs in German camps (I think around 6%? Could be wrong here).

Sandra Wilson, Robert Cribb, Beatrice Trefalt, and Dean Aszkielowicz in Japanese War Criminals: The Politics of Justice After the Second World War have jointly suggested that the majority of atrocities arose when the captives were inconvenient (similar occurrences happened on all fronts, and included Axis POWs), the neglectful treatment of POWs, and punitive counter-insurgency raids. Examining the second option, the neglectful treatment of POWs, my own research is showing that in the initial stages of the war with Japan, the treatment of Allied POWs was relatively good. The labour was mostly camp related, such as chopping firewood, tending gardens and carrying water, and the rations, while not in large supply, was relatively good. Indeed, reassessments of the Changi POW camp show that during this stage, recreational activities such as cricket, boxing, and rugby were mainstays of camp life (Kevin Blackburn, The Sportsmen of Changi). The main turn around occurred in the latter half of 1943, where many prisoners saw and received an increase in beatings, summary executions, and a steady decrease in rations. I am proposing in my thesis that this occurred for several reasons;

In late 1943, the war had begun to change for Japan. The submarine fleet of the USN had begun to wage a truly effective unrestricted campaign against Japanese shipping. This led to a massive decrease in supplies for Japanese garrisons, many of which were in isolated Pacific and Asian island outposts. This of course led to a decrease in the food and medicine available to the POWs, and the loss of shipping likely led to "reprisals" by guards against the POWs, who represented the enemy to them. The Japanese military had a tremendous number of casualties that died not because of enemy action, but rather lack of food and medicine. One source (which I can't remember right now, but will add later) argues that approx 50% of Japanese casualties was due to inadequate food and medicine, rather than battlefield deaths. The placement of many camps in malarial areas certainly did not help.
Many of the camps that held POWs were meant to be temporary. This was because the massive number of POWs that Japanese captured was not expected. However, many POWs were never moved, and the initial forces who captured the prisoners, often well-trained and well-disciplined front line units, were slowly replaced by poorly trained, poorly disciplined Japanese colonial troops from Korea and Taiwan. Several authors, and even former POWs, argue that these colonial troops, themselves subject to beatings by their Japanese superiors, often vented their anger on the only available outlet: the Allied POWs. To look at the problems faced by large numbers of surrendering personnel, several thousand Wehrmacht prisoners died in the final months of the war in US custody because the US army was simply unprepared to house, feed and tend to some 300,000 soldiers captured extremely quickly ( Rüdiger Overmans, “German Historiography, the War Losses, and the Prisoners of War”, in Günter Bischof and Stephen E. Ambrose (eds) Eisenhower and the German POWs: Facts Against Falsehood).

Many authors, particularly in the post-war memoirs of the 50's and 60's attribute the large number of deaths to Japanese culture, particularly the way of Bushido. Examining the above information, however, makes me think that this is wholly inadequate, and probably the result of latent, though not always intentional, racism of the era (John Dower,* War Without Merc*y has some stuff about this).

What my answer doesn't cover is the massive deaths that occurred in China. While I can't qute remember where, I believe either Yuki Tanaka Hidden Horros: Japanese War Crimes in World War II or Peter Li (ed) Japanese War Crimes: The Search for Justice attributes many of these deaths to Japanese soldiers asserting themselves over the Chinese. Prior to the Japanese war (intervention, whatever you want to call it) in China, China was often seen as the more powerful of the two countries, and Japan in some respects was seen to be the "little brother" of China. The war crimes committed in China were Japanese soldiers realising this was no longer the case, and flexing their new found dominance. I think tat a more convincing argument is that the Sino-Japanese War of 37-45 was often a guerrilla war, one that has also shown to lend itself towards brutal atrocities against civilians, largely due to intense pressure and PTSD (think the Americans in My Lai and other such atrocities, as well as German reprisals in Eastern Europe (Hostages Trial from the Subsequent Nuremberg Trials, 1948)).

I apologise for the litany of spelling, formatting, and grammatical mistakes. My supervisors were not here to hold my hand, so I probably done goofed hard.

Edit: This is in no ways meant to be an apology for, or excusing, Japanese war crimes. What happened was truly abhorrent. I mean only to provide some context for how such crimes occurred.