I'd say Geneva Convention alone was reason enough to not give them any terms of surrender   
Honestly, China wasnt going too well for them.
War of attrition going into Chinese heartland with no infrastructure.
I think they took action much earlier.
I remember History channel mentioning that they were basically half mercenary half US special interest at the time

WW2 was a war for lebensraum, both for Germany and Japan.
Both were industrializing rapidly and was facing resource problems in terms of what to feed the factories and people.
Manchuria can solve the factory problem, but cannot solve the food problem.