The destruction of Alderaan is often cited as ipso facto proof of the Empire's "evilness" because it seems like mass murder--planeticide, even. As Tarkin prepares to fire the Death Star, Princess Leia implores him to spare the planet, saying, "Alderaan is peaceful. We have no weapons." Her plea is important, if true.
But the audience has no reason to believe that Leia is telling the truth. In Episode IV, every bit of information she gives the Empire is willfully untrue. In the opening, she tells Darth Vader that she is on a diplomatic mission of mercy, when in fact she is on a spy mission, trying to deliver schematics of the Death Star to the Rebel Alliance. When asked where the Alliance is headquartered, she lies again.
Leia's lies are perfectly defensible--she thinks she's serving the greater good--but they make her wholly unreliable on the question of whether or not Alderaan really is peaceful and defenseless. If anything, since Leia is a high-ranking member of the rebellion and the princess of Alderaan, it would be reasonable to suspect that Alderaan is a front for Rebel activity or at least home to many more spies and insurgents like Leia.
Whatever the case, the important thing to recognize is that the Empire is not committing random acts of terror. It is engaged in a fight for the survival of its regime against a violent group of rebels who are committed to its destruction.
