If you read Japanese history books you might discover that Japan loves to do sneak attacks. They lull their opponents asleep and attack when they least expect it. And when they invade a country they wreak as much destruction as possible. What happened in the 30s and 40s was just a pattern of behavior the Japanese utilized before for countless centuries.
That's pretty much the standard non-Western way of war. They don't declare war all that much. The Chinese intervention in Korea was a massive sneak attack, but no one's ever called them on it. We lost thousands of GI's that day.
And destroying the enemy's stuff is also pretty standard issue military strategy. We killed more people via strategic bombing in Germany than in Japan, and burned their cities to the ground with napalm. A lot of this stuff is meant to cow the populace in order to minimize the resistance to their rule once they win. Killing millions of Japanese and German civilians via aerial bombing certainly helped secure the peace once we defeated their conventional forces. The underlying threat was that we would pull back in the case of rebellion and burn their cities to the ground again. Between the chevauchee during the Hundred Years' War and the depredations of the Thirty Years' War, I think it's hard to say that the Japanese have any monopoly on medieval military practices. The Chinese themselves haven't exactly been angels. In the 18th and 19th centuries, they exterminated the Dzungars, massacred tens of thousands of Taiping rebels after they surrendered in Nanking and did another post-surrender massacre of an estimate 70,000 surrendered rebels in Yunnan. Bottom line is that nobody has clean hands.