I know of course that no nation or its people can be summed up as briefly as is done in the title, but given the character limit I unfortunately had to make a slight generalisation, for which I apologise!
And yes, of course I'm not trying to say that other nations weren't just as brutal, I'm only specifying Japan because their culture seems to be so at odds with their military at the time.
To be a little more in-depth as to my enquiry:
During WW2 it's well known that the Japanese military were *ruthless*, even compared to other allied and indeed axis powers. I won't go into specifics as I'm not a historian myself and don't want to give any misinformation, but to briefly name a few examples of the brutality to which I refer, I'm specifically thinking of their brutal torture and mutilation methods and their lack of regard for even their own citizens.
This, when compared to the Japanese people and their, deserved, reputation for being overwhelmingly friendly, respectful, cool-headed and polite. A reputation I believe they held at the time of WW2 as well.
I apologise if this post is offensive, it's not my intention to be disrespectful, I am admittedly rather ignorant about this matter and I'm more than willing to be corrected. Thank you!
/u/ParkSungJun in this answer deals with the structure and culture of the IJA which led to massacres such as Nanjing during the 2nd Sino-Japanese War. Here is another answer from the same user expanding on certain aspects of the question.
This answer from a now deleted user touches on what changed between the Japanese Army's relatively decent conduct towards POWs during the Russo-Japanese War vs the 2nd World War.
Of course, plenty more can be said on the subject.