Let's just start with "New Hope" for a second and examine how the "non-oppresive" Empire operates.

First, Vader's star destroyer captures the ship Leia is using to escape/transport plans for the Death Star. Vader asks the Captain where she is, the Captain insists they're on a diplomatic mission (he may well have had no knowledge that Leia had the plans) and isn't forthcoming with Leia location, so Vader breaks his neck. Great interogation technique there.

So Leia gives the plans to R2D2 and C3PO, and the droids escape to Tattoine, and the empire follows them there. The Empire finds the Jawas that took the droids, and after finding out who the Jawas sold the droids too, kills all of them. Why? What purpose did it serve? They also attempt to make it look like Tusken Raiders were responsible for the massacre, shifting the blame away from themselves.

The Empire then goes to the home of Luke's Aunt and Uncle, and after finding out that the droids are no longer there, kills Owen and Beru and destroy the farm. Again, why? What possible tactical purpose does it serve other than attempting to inspire ear and keep the rest of Tattoine's residents in line and subservient to the Empire.

Now, let's look at the Death Star itself, as conceived in "New Hope." It's an instrument of terror, not a means of defense against the rebellion. Near the beginning of the film, Grand Moff Tarkin tells the Empire's other admirals that the Emperor is disolving the Senate to take total control. And how will he rule? "Fear will keep the local systems in line. Fear of this battle station."

Then the Empire uses the Death Star to destroy Alderaan, a peaceful defenseless planet. They decide not to use it against the military target that Leia gave them, because, in Grand Admiral Moff Tarkin's words, it's "too remote to make an effective demonstration." So again, the Empire is blowing up a peaceful, defenseless planet as an object lesson for the other systems in line and living in fear that it could happen to them. Life doesn't sound so damn peachy under control of the Empire, does it?

The bottom line is the Empire was a trynical goverment that ruled by fear and terror. They conducted war under the guise of trying to preserve peace. If you side with the Empire and the way they conducted, you're basically siding with the Nazis.