But. BUT. Lets take a look at a similar situation. Lets look at the United States. Let's take a look at the military; the tanks, the guns, the soldiers. Military more often than not is meant to invoke fear, to invoke respect. You sure don't want to dress your army in clown costumes. You want your enemysenemies to fear you, not laugh at you. Now, imagine for a moment that all you ever saw or knew of the US was its military. The guns, the airplanes, the soldiers with their sunglasses and kevlarKevlar. It would seem pretty terrifying. That's all you would ever picture when you heard the name "United States". Pure destruction, and terror, and military. You would have no idea that these soldiers are probably regular people like you and me. That singular, narrow view of the US would probably make the US seem a bit evil.
So, going right on back to the Empire, does the "Imperial" see itself as evil? Well, what is the Empire? If I'm up to date on current lore, the Empire phased out the clone program and started recruiting from the multitudes of planets that are part of the Empire. So, the "Imperial military", while headed by the insidious Darth SiduousSidious, is, mostly, your average Imperial citizens. Each of those nameless, aim-less Stormtroopers cut down by Luke probably has a family, or a brother or sister (or some sort of alien-relation). And each of them probably had no idea that Darth SidousSidious was a Sith. Even if they had, would it have mattered to them personally? Their families probably had food on the table every meal, had a house to sleep in, had an Imperial check arrive every month to put in the Imperial Bank. They didn't go to bed every night worrying that their supreme leader was a wrinkled Sith. In fact, the idea of Sith and Jedi had become pure legend and witchcraftery, if look at the Vader's first scene on the Death Star. Sure, the average Imperial Citizen might hear news flashes every now and then about "Rebel Attacks on our Empire's Fleet" or "Terrorist Bombing of Imperial Star Station", but as long as they're not affected, they probably didn't care. What would you do if today you learned that your country had been headed by a Sith for the past 20 years?