This has been discussed earlier in:

What changed in the Japanese military that caused them to be so brutal in WWII after their mostly professional behavior in the Russo-Japanese war?

The short answer is

The behavior of Japanese troops towards civilians was very much a balance between the Army's intrinsic atavism and the degree of control exerted by higher authorities over it; this balance was different in 1905 vs 1938. In 1905, Japanese leaders who cared about international perceptions had the upper hand; in 1938 they didn't.

. . . for a full discussion see the answer to the earlier question.