We do not know enough of the Empire to say it's good or evil just because. We accept it at face value because Darth looked slick at the time and those effects - yup, must be evil. They all have British accents too and this is largely an American flick so some out there will parallel the OT with the American war of independence, lamentably so... (Insert here enough rolleyes emoticons to cause internet bandwidth to crawl to standstill.) Seriously, did anyone really think that with any sincerity?
Never once did I equate Star Wars with the American Revolution because the Imperials seem to favor a British accent.
There is plenty of on-screen evidence to support the fact that the Empire is evil. At least the leadership of said Empire is evil. And this was even before the prequels.
ANH: Vader chokes the life out of the captain (later revealed to be Captain Antillies) of the Rebel ship (later revealed to be the Tantivie IV). We find that the Emperor (later identified as Palpatine) dissolved the Senate, thus turning the democratic or republic style of government into a true dictatorship controlled by a totalitarian autocrat who has placed evil twisted men in positions of power and authority. We can surmise this placement was in order to have loyal supporters of the Emperor's regime. The Empire designed a weapon of mass destruction capable of destroying an entire planet. Tarkin had the power and exercised his authority to destroy an entire planet. While an argument can be said that the planet was in rebellion against the Empire, there was no evidence to support this. He condemned an entire planet based on the actions of a single starship and one politician from that planet. Said planet was not even an opposing military force or nation but was comprised entirely of Imperial citizens. Later Tarkin orders the execution of Princess Leia without due process or fair trial. Imperial troops had the freedom to slaughter innocent Jawas and 2 Imperial citizens in a quest to capture 2 droids. The Jawas and the Lars family posed no threat to the Empire.
ESB: Most of the fighting is actually legitimate use of governmental authority against people in open armed rebellion. The torture of Han Solo, however, is evil. They didn't even ask a simple question. Torture for torture's sake.
ROTJ: Really can't think of anything evil that hadn't already been established. Many Bothans died acquiring the new Death Star information. We lack details as to if they were civilians, if it was a whole city or planet, or what. Perhaps the Emperor's attack on Luke could be considered evil.
When we add in the subsequent movies, we get an even broader picture
TPM: Palpatine, in his guise as Darth Sidious, orchestrated and manipulated both sides of the Separatist conflict and he did this to suit his own megalomaniac agenda for power. The implication (I've not read any subsequent tie-in material that contradicts this) is that Palpatine impregnated Shimi Skywalker without consent. The catastrophic death toll on Naboo is a result of Palpatine's meddling.
ATOC: Palpatine uses the emergency he caused and continues to manipulate to seize power.
ROTS: Palpatine ignores due process and the legal authority of the Jedi to remove him from office. Palpatine uses a hypnotic pre-programmed order to force troops to turn on their allies. Palpatine orders Anakin to murder the very Separatist leaders that Palpatine had been manipulating for years in order to cover his tracks. Palpatine orders Anakin to murder innocent children. For years after Anakin is used to hunt down and murder Jedi.
R!: Krennic is not above using lethal force to persuade, capture, and manipulate the Erso family. Abduction and imprisonment is a viable means to achieve his goals. Krennic orders the execution of innocent scientists who may also have been prisoners of the Empire. Krennic and Tarkin willingly order the destruction of an entire city simply to test a weapon of mass destruction. The people of Jedha are implied to have been used as forced labor in construction of the Death Star.
SOLO: Imperials are seen as being corrupt.
So the evidence is overwhelming that those in positions of authority within the Empire were quite evil. Palpatine, Vader, Tarkin, Kerric. All sick evil men that were comfortable in their use of lethal force, blatant murder, and other crimes against humanity to perpetuate and support the government.
Other Imperials were clearly just doing their job. Most of the Imperial solders, even though the slaughter of innocents like the Jawas and the Erso and Lars families are black stains that should have caused the soldiers to refuse orders. Admiral Piett seemed to be a simple soldier following orders. Even Moff Jerjerrod seemed like a loyal soldier overseeing the construction of the Death Star and not a war criminal, unless one holds the view that creating said weapon of mass destruction is enough to label him evil.
