So, firstly, its important to understand the context of Japan's former treatment of POWs. Their first foray into the international community as a 'major player' so to speak, was in the Russo-Japanese War prior to WW1. Rotem Kowner writes about this particular topic in an article for The Historian, "Becoming an Honorary Civilized Nation: Remaking Japan’s National Image During the Russo-Japanese War 1904-1905". Kowner points out that the Japanese were viewed as feminine and weak internationally and had to put on a good show for the world, so they were exemplary to gain clout politically and be viewed as strong (Theodore Roosevelt commented on their bushido warrior spirit, saying "he Jap [who] goes in to return no more forever, and such an end is his wish", quoted from Kowners article).
So, we have them presenting a False Face to the world so to speak. But what happened between the Russian conflict and WWII, to your question?
Nationalism. Nationalism happened, and a bastardization of Bushido. Daqing Yang, in a compilation of essays available on the national archives entitled "Researching Japanese War Crimes", comments about the Rape of Nanking and how the infamous horrors of the mainland invasions by Japan were a test run of sorts for the Japanese Imperial Army to utilize excessive military doctrine in combat. In Yuki Tanaka's famous "Hidden Horrors" book about WWII war crimes, she gives an example of Japanese enlisted men needing to behead Chinese captives to even become an officer during this time, showing the dehumanizition of the enemy become standard.
Further, Philip Towle has a collection of essays entitled "Japanese Prisoners of War", wherein an essay by Susan Townsend points out that in the 30s, there was a cabal of political and military elites seeking to take all of Eastern Asia for the Yamato race (the East Asian Co-Prosperity Sphere, for those not in the know). They believed Eastern morality was innately superior, their genetics were superior, and they sought to supersede and usurp all other Asian races.
Constantin Vaporis has an idea that bushido never truly existed as a cohesive idea until roughly the time of Inazo Nitobe and his book on the very topic, "Bushido: The Soul of Japan", in 1899, regardless of it being a cultural ideology for 600 years just not codified. But, bushido was the main component of the Imperial Army and their rank and file pamphlets for WWII so what gives?? Vaporis says that rigorous subservience to the emperor was simply something that could be a political tool and modified to the needs of the army. Karl F. Friday agrees wholly with Vaporis in his own "Bushido or Bull" article, calling it extensive propaganda and pointing out that literally not a single example of medieval bushido demanding dying for ones lord exists, and any instance of such would have had other pragmatism backing such an occurrence. So, the WWII Imperial Army just subverted and twisted it: "The modern injunctions against surrender and capture were probably developed as a way for a country like Japan…to squeeze the most usage out of its troops." per Friday.
Let's combine the points: bushido is a cultural idea since the 1200s in Japan. Japan begins to bring it back around 1900, has a war with Russia in 1904, puts on a good show for the western world and that lasts through until right before WWII and the Chinese territorial incidents. The cultural and military elite, at the same time, have an idea to take over all of East Asia because they're superior due to being the Yamato race. The military begins to ramp up the bushido spirit and brainwash their rank and file, making atrocities the bare expected minimum: this creates a culture of 'death before dishonor', and every soldier expects to die at the enemy's hand so why would they offer any better? Also, logistically speaking, they didn't have the troops and materiel to support prisoners, they could scarcely support their own! Friday really sums it up best, they were squeezing the most out of what they had, and that meant disposing of morals.
I can discuss further upon request!
Bibliography
Constantine N. Vaporis. “Inventing the Way of the Samurai: Nationalism, Internationalism, and Bushido in Modern Japan.” AMERICAN HISTORICAL REVIEW 121, no. 1 (February 1, 2016): 224–25.
Edward Drea et al, Researching Japanese War Crimes, www.archives.gov/files/iwg/japanese-war-crimes/introductory-essays.pdf
Karl F. Friday “Bushidō or Bull? A Medieval Historian’s Perspective on the Imperial Army and the Japanese Warrior Tradition.” The History Teacher 27, no. 3 (1994): 339–49.
Philip Towle, Japanese Prisoners of War. (London: Hambledon Continuum. 2000)
Rotem Kowner, Becoming An Honorary Civilized Nation: Remaking Japan’s National Image During the Russo-Japanese War of 1904-1905. The Historian 64 no. 1.
Yuki Tanaka, Hidden horrors: Japanese War Crimes in World War II. ROUTLEDGE 2019.
For further reading to support my statements herein, read: John Hickman. “Explaining the Interbellum Rupture in Japanese Treatment of Prisoners of War.” Journal of Military & Strategic Studies 12, no. 1 (Fall 2009)