I don't quite agree with the initial point - this is fantasy, not historical fiction, not reality - so for the most part I assume that all we are shown is true. If we are to assume otherwise it all becomes somewhat pointless - you might as well write a new movie.
The unreliable narrator is a common element in fiction.
From what we are shown, the basis for the assertion that Palpatine is a villain is sound. Neutral Evil, I would suggest.
But even were I to agree with you, I certainly find myself hard pressed to find a Neutral Good reason for the destruction of Alderaan.
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Every indication in Ep IV is that the Alderaan is peaceful (and not at war with the Empire) and the Empire did not hesitate to destroy it, ostensibly as a 'test' for the Death Star's capabilities.
What are we actually shown about Alderaan? We are shown that the highest official of the Alderaan government (Bail Organa was the First Chairman and Viceroy of Alderaan) is a supporter of the largest known terrorist organization in the Empire. We are shown that another member of the government (Leia was a Senator for Alderaan) was an active member in the same terrorist group. We are shown that officers on simple diplomatic ships from Alderaan are armed, trained, and willing to kill Imperial soldiers rather than submit to a simple and legal search of a ship.
If we expand to the radio play and novel version of ANH, we are also shown that the Alderaanian government assassinated an Imperial official who started to suspect the Viceroy of being Rebel, and that the murder was covered up as a "hunting accident". We are also shown in RotJ that the same ship used by Princess Leia in ANH makes up a significant percentage of the Rebel fleet, implying that Alderaan was supplying a terrorist group with warships.
What we are not shown is any evidence of Alderaan's "peaceful" nature. Literally every Alderaanian we see is member of a terrorist group, and Bail Organa is the only one we see who isn't killing Imperial troops. The only evidence we have that Alderaan is "good" in any way is Princess Leia telling us that. And I for one don't trust a traitorous senator who feels no guilt about using lethal force against attackers that she knows are using non-lethal weapons.
Every indication in Ep IV is that the Alderaan is a violent terrorist planet that the Empire was forced to destroy for the good of the galaxy.