yeaaahhh...they were entirely justified.
If you believe what you posted, you don't know jack about the subject.
The Japanese would not quit the fight when it was obvioous they would lose. Were given ample opportunities to surrender honorably, but chose to continue the fight.
Additionally, the Japs didn't honor any conventions regarding the conduct of war and treatment of POW's.
Furthermore, the Nips ruthlessly killed Chinese civilians as a matter of policy for over twenty years before the attack on Pearl Harbor occurred.
Finally, their barbarity had no limits. Many Jap field commanders were sadists and cannibals. They would routinely beat prisoners for no reason. They would eat the liver, heart, and other parts of prisoners...they believed it gave them "special powers".