We're not talking about whether or not the Empire is breaking the law or not (their own laws.. apparently they aren't). But that makes no difference. We're discussing the morality of their decisions (hence the term "evil" which is a moral judgement).
Legal does not always equal moral or ethical.
Now I will argue my reasoning as to why the events are not so similar as they might appear:
Also, it can be argued that their actions were BAD for society. Despite the effect: killing a few alleged terrorists (without trial or evidence being presented I might add, but their laws are different from our's) and scaring the rest of the civilized galaxy...
They still encouraged (and failed to stop) the rest of the rebellion from happening. Its almost completely obvious that the creation and use of the Death Star only promoted the idea that a rebellion was necessary, and galvanized those who already had a dislike of the Empire and its methods. So in a sense, the Empire brought this upon themselves... since we know that the Death Star existed as a concept BEFORE the Empire took power, not something that was whipped up in a hurry because of the evil Rebels suddenly attacking the government.
What I'm saying is that, while you think that nobody can use the U.S. as an example, when they vaporized a whole city with millions (or thousands, whatever) of inhabitants in Hiroshima, because you don't condone it and still think it's evil, you argue that the Empire would be too evil to do the exact same thing.
It's just a matter of scale. The U.S. is a country, with several cities, as is Japan. The U.S. decides to make two whole cities disappear in a big bang (as a demonstration). The Empire is a galactic civilization, with millions of worlds. They decide to blow up one (as a demonstration). I'm sure that during the middle ages when wars were fought with swords and arrows, the concept of blowing up an entire city with a giant fireball would have seemed immensely evil.
Scaling it down sounds like a plausible argument for relativism (and therefore dismissal) at first, but here is where it falls:
There are billions of people in our world right now. So therefore, it doesn't matter one lick, if I kill one (or a dozen) people right now. After all, there are billions more, and thousands die every minute from accidents, disease, old age, etc. So therefore, murder is okay.
Then you have chaos, because if everybody thinks that way, lots of people die (but again, you could try to argue good comes from it... more resources for everyone else).
It's ludicrious because murder is murder. Murdering billions is more evil than murdering millions, which is more evil than murdering thousands.
So:
USA: Murders thousands < Empire Murders Billions/Trillions (?)
?!
Was Alderaan a military target? Both Leia and Tarkin seemed to feel it was not.
The USA's action is still evil (murdering thousands of civilians), their excuse being that it ended the war, made other countries afraid to mess with them (well not really... Cold War 'nuff said) and that it was the lesser of two evils. Apologists for the nuking of Japan would have to argue that it was a lesser evil, to force Japan's surrender, than to force them to surrender by continuing the war in conventional fashion.
Despite the evilness of the USA's actions, it is utterly different than the (much more evil) actions of the Empire.
The atomic bombings were NOT done to weed out terrorists within US territory. They were done to terrorize the civilian population, in order to crush the will of Japan, so they would be forced to surrender. A comparable scenario would perhaps be if the USA nuked New York, until the entire state was vaporized, in order to wipe out a handfull of known Al-Qaeda operatives that were suspected of hiding out there. Scaled up of course... did all the innocent people deserve to die simply because they lived nearby somebody who may have been guilty?
The people who died in Japan were mostly civilians, not soldiers, or terrorists. It was done for an entirely different reason. The only similarity is that a lot of innocent people were killed. The atom bombings "worked".... Japan stopped attacking, the war ended.
The Death Star incident "didn't work." Sure it stopped whatever supposed terrorists or funding that came out of Alderaan (except princess Leia and whomever else escaped with her), but it made the rest of the galaxy churn out enough rebels to destroy the Death Star, and eventually overthrow the regime.
Clearly the Empire had more options available to it than the US did. The US could have backed off, or had a land invasion of Japan (leading to more casualties on both sides.. though they would have been more military personal proportionally you'd guess). But that's about it...
The Empire could have easily set down a strike team to take out the "terrorists" on Alderaan. They could have blown up an uninhabited moon or something as a demonstration or test of the DS.
They could have ordered and orbital bombardment of key targets (which would have been murderous, but not on the scale of killing everyone on the entire planet and anhilating it).
The Death Star incident puts the other evil actions of the Empire to shame. Though, if we are to believe the EU, they constructed other more vastly destructive terror weapons as well (Sun Crusher, Galaxy Gun), so I'd say they wore out whatever mandate they had, and whatever moral right they had to be in power. Note that these weapons were not contructed to deal with any external threat (as the nuclear weapons of the USSR and US during the cold war were... ie: to use against each other if the other power attacked) but to use against THEIR OWN POPULANCE!
If the US stockpiled nukes to use against its own cities, then maybe they'd be more Empire-like.
Now that we've mentioned stability (I guess many dictatorships and totalitarian regimes have done some good things when they're not oppressing the masses or committing atrocities)...
The Empire is hardly what I'd call stable... they have a full fledged rebellion on their hands (that they seem to have caused), that succeeded.
I suppose now I'll be told that the Empire was "fine" and that the Rebels were just rebelling for no reason. Giving their lives just for the heck of it... rather than because the regime they lived in threatened or oppressed them!