from GURPS Space, First Edition, by Steve Jackson and William A. Barton, © 1988 Steve Jackson Games
It's nice to think that a government that Goes Too Far will eventually cause the citizens to rise in righteous wrath and throw the rascals out. It's also convenient when all the defenders of the Evil Regime wear uniforms (except for the occasional Secret Police spy). Unfortunately, we know from centuries of experience that it doesn't really work that way. The worst tyrannies imaginable have been enthusiastically supported by people no worse than you or I.
Without going deeply into psychology, here are some of the reasons citizens support tyrannies, which you can use to make your fictional Evil Empire and its people something more than laser fodder:
Citizens fear the unknown will be worse than the known: a foreign philosophy, a strange religion, society breaking down to anarchy. They may fear and hate an enemy population, especially if they are a different religion or race. Let alone species: do you hate the Bug soldiers because they are cruel and ruthless, or because bugs are icky? Many people fear that a new government would cost them their jobs or personal power; in a corrupt regime, they may have good reason to be afraid of justice. A clever regime's propaganda will play on all these fears, constantly portraying the foe as inhuman, the rebels as terrorist killers.
People who are used to obeying the law often have a hard time changing their habits when the law becomes oppressive. They still believe that "the police only arrest criminals; honest people have nothing to fear." When the rebels break into an armory to get guns, these people only see that a robbery was committed. Enough of this, and patriotic citizens may volunteer for the army to fight the wicked rebels. Obviously, rebellions find more support on worlds that were free until the empire conquered them. But even there, some citizens may hate the occupier but doubt that the rebels would be any better. You can fight for "freedom" -- but once you win, you have to set up a government.
And people may be loyal to the idea, or even the ideals, of a nation or empire, even when the reality is tarnished. "My country, right or wrong . . ."
It is not evil, or even cowardly, to be afraid of starvation, torture and death. Any successful rebellion must overcome these fears . . . to convince the people that anything is better than slavery. Meanwhile, the government is telling them that anything is better than anarchy. Which is why rebellions have a hard time of it.