All the sides in WW2 were awful. The only major participant (unless you deem them as minors) that didn't have blood on their hands was arguably Greece and Finland.

Japanese (arguably the worst faction in WW2 and barely punished)

• Multiple genocide events in China. Minor ones throughout Austronesia and Polynesia.
• Sexual enslavement of both Asian and non-Asians. Even people from within their own Empire, such as the Koreans, were sexually enslaved too.
• Enslavement of PoWs which is against Geneva Conventions.
• Bayoneting and mass killing of medical workers in places like Hong Kong and Singapore.
• Beheading competitions in places like Nanjing.
• Biological and chemical experiments on civilians
• Allegations of them trying to unlesh biological weapons on USA
• Convincing young men into suicide planes and only a few number of them actually made an impact. American mines sunk more Japanese ships than Kamikazes sunk American ships. The ones who bailed out last minute were still shamed and ostracized after the WW2.

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United States and British Empire:

• Starvation of millions in Bangladesh
• Concentration camps for Japanese but not for Germans / Italians
• Firebombing cities throughout Germany and Japanese. Postwar defence: "we did it to stop their industry durrrr!" except Germany production numbers always went up in WW2.
  
• Nuclear bombing 2 cities in Japan. "We did it so they would surrender!11!" even though Japanese sources put the loss of Manchuria and Korea ahead of these nuclear bombs. The firebombings were more destructive and killed more than those 2 nuclear bombs so the nuclear bombs looked minor.
• American soldiers rarely took Japanese as PoWs.
• Quite a few documented cases of Americans, British and Canadians massacring surrendering Germans too. It's why Patton was temporarily punished and prevented from joining Operation Normandy because troops under his command did massacres. Out of the 3 factions: it was said the Germans preferred surrendering to Americans because they were afraid of surrendering to British and especially Canadians in particular.
  

Soviet Union

• Genocide of millions of Ukrainians a few years before WW2.
• Ethnic cleansing of Tartars, Chechens, Germans, Scandinavians, Dutch, Poles, et cetera.
  
• Widescale arrests, killings and deportations of both national and non-national civilians.
• Turning a blind eye to military rapes.
• Enslavement of PoWs.