It is well known that the brutal Japanese treatment of enemy combatants and POWs during the Pacific War 1941-45 was well beyond the exact opposite of the their humane treatment of Russian combatants and POWs from the 1905 Russo-Japanese war.
The reasons for the change have been discussed elsewhere on this forum, without any clear reasons being established.
Here is a very sound reason from Gerald Linderman’s The World Within War: America’s Combat Experience in WWII, Harvard University Press, Cambridge (Mass) 1999 paperback edition, p.153 and elsewhere.
Linderman argues that that Japan placed the treatment of POWs within the realm of national policy rather than moral, humanitarian or other concerns.
In 1905 Japan was trying to impress the West with its achievement of Westernisation: military, industrial and behavioural.
By 1941 Japan was bitterly angry with the Western powers which oppressed Japan and was determined to expel them from Asia. Any elements of respect for, alignment with and emulation of Western values had been rejected in favour of contempt and hatred for Westerners.
Linderman doesn’t quite say this but, combining those attitudes with the fairly medieval military attitudes and practices extolled by the militarists in their synthesis of contemporary ambition and corrupted traditional values, this laid the groundwork for the abandonment of all Western combat and other war rules and for the mistreatment of Allied POWs.
There is a lot more to it that is brought out by Linderman, but the elements I’ve outlined go a long way to explaining why the Japanese did an about face between 1905 and 1941 (really 1931 starting in China).
Which goes back to the 1921 humiliation of the Japanese at the Washington naval treaty talks.