I know Inperium can’t be achieved through being nice about it but many of the tactics used by the Japanese, particularly in the Massacre of Nanjing, Pearl Harbor and the Pacific Theatre manage to make Hitler seem tame by comparison. Why did Japan commit war crimes like Nanjing and somehow enjoy it like they were Mongols?
We've had several threads addressing the behavior of the Imperial Japanese Army, as always, there's more that can be said, but you might start with the discussion at:
and
I would observe that your question has an assumption -- "insanely cruel" ; it's not helpful to start your inquiries with an assertion of irrationality. If we say that people are insane, then there's no historical explanation for their acts. War is naturally "insanely cruel" in many ways . . . the Japanese behavior during WW II was cruel in a fashion that European military traditions regarded as illegitimate and illegal, but was it "insane"?
Cruelty is often an assertion of power with a practical purpose; if one looks at, say, Timur - he seems to have consciously sought to equal the reputation of Chinggis Khan . . . that is, his terror seems at least in part to have been calculated. That's often the case-- terror need not be "insane", sometimes it's a deliberate strategy. In the case of the Japanese during WW II, their atrocities seem to have been a mix of intentional policy "suppress the bandits" and a natural atavism unconstrained by military discipline.
Similarly Chinggis Khan himself benefited by the flood of refugees fleeing ahead of his army, warning people "don't resist the Mongols"; they were "cruel" yes, but there seems to have been a purpose behind their actions.
See:
A. Azfar Moin, The Millennial Sovereign: Sacred Kingship and Sainthood in Islam, (Columbia University Press:2012) for a discussion of the political purposes of Timur's cruelty.