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.
Discuss.
Old, but amusing, Robot Chicken video below the fold.
Bonus Video added by Ace, too.
Obi Wan attempts to buy a used car. Hidden camera foolishness with a Jedi Master.