Well, I happened to be native Japanese.
Simply saying, the answer is because the relationship after the WW1, Japanese Imperial Army aggressively started invading China ( Second-Sino War )., which had them expanded so deep into China, whereas, the U.S and its allies were pressuring more and more on Japanese about it. ( Even Imperial Japan relied heavily on the production of critical materials such as iron through the import from the U.S.
According to here, even at the time of the attack on Pearl Harbor,
the difference of the materials produced between Japan and America is as below.
GDP : the U.S 12.7 vs the Imperial Japan 1 ( hereunder same )
Annual volume of production of warships 4.5 vs 1
Airplane 6 vs 1
Raw iron 10 vs 1
Coal 10 vs 1
Electricity 6 vs 1
Oil 740! vs 1.
Due to the huge gap? as well as the difference of opinions on Chinese matters, Japan had become callig Americans and British in Japanese "鬼畜米英", translated, "Americans, British The Fiend".
Probably the most infamous treat by the Japanese Imperial Army of American POWs is Bataan Road Deathmarch, which the Imperial Japanese Army estimated the numbers of POWs should've been 15,000 〜 20,000, whereas the actual number was 80,000. From the start, Japanese Army themselves are not fed enough ------------------
So that, when the Imperial Japanese Army, poorly equipped and thousands of themselves died out of hunger themselves during the Pacific War, it is easy for us to consider that Japanese Imperial Army treated American, or British POWs brutally so. ( But I don't think it is no comparable to Holocaust by Nazi Germany though...... but well, the defeated can not say so much. )