Their feudal system treated their own peasantry badly. Non-Japanese are not considered human. Worse, surrender meant you're the lowest of the subhumans. Killing them or POWs was like killing varmints. Are you going to treat prisoners, who surrendered, better than your peasant class? Japan and China have centuries of conflicts between them. Centuries of animosity certainly did not help. The result is terrible medical experiments (example, microwaving POWs to death), forced death marches, slavery til death, rapings, practiced executions (like we practice hunting varmints), and so on. Their history doesn't make their participation good or bad. It simply is their history and culture. Not all cultures are equal. It's important we weigh history through the lens of culture. At least enough to understand. Understanding doesn't, in of itself, make it right or wrong.