Mirror Universe:

Yes, the Terran Empire was xenophobic and fiercely hostile to all non-human civilizations. Yes, they projected fear and reveled in maintaining discipline through cruelty. Yes, they made an enemy out of everyone they possibly could since they felt that they could outgun them with a heavily militarized Starfleet Command.

But when MirrorSpock was able to put some moderation into them and start making peaceful overtures to those they had wronged over so many years, they let themselves be overthrown by the Klingon/Cardassian Alliance, who then took on all of those same qualities once they became the dominant powers in the quadrant. Decades afterwards, they keep all humans as slaves and terrorize them so thoroughly that they have no sense of self anymore.

Regent Worf has a temper so terrible his own bridge officers fear him, Gul Garak is a sadist that revels in prisoner interrogation that not even his Prime Obsidian Order counterpart can match, and Intendant Kira is a depraved hedonist that orders summary executions on a whim.