Honestly, it was the destruction of Alderaan that led directly to the Empire's downfall. Most people were willing to go along with the idea that the Empire would keep them safe, but after Alderaan, people starting questioning that, including many in the Imperial military who would end up defecting. It was a major miscalculation in a way, though I don't think the Emperor cared about keeping up appearances at that point, with the Death Star online. He dissolved the Senate immediately and made his "unstoppable" battlestation the symbol of his power. If only they'd caught that exhaust port thing....