Just because Anakin Skywalker turned to the Dark Side didn’t mean he loved the Empire. In fact, there are signs he may have been subconsciously subverting the Empire during the entirety of the original Star Wars trilogy and even before. In many ways, his actions resembled that of a fifth columnist: doing terrible things in order to gain the trust and leverage he needed to aid the Rebellion just when they needed it. He may have done so subconsciously or without awareness – the Force moves in mysterious ways – but it’s clear that he secretly hated the Empire the whole time.
In fact, hatred of the Empire actively enhanced his Dark Force abilities, since it gave him a constant and immediate target for his negative emotions. He told Palpatine how much he’d like to kill his soon-to-be master in Star Wars: Episode III – Revenge of the Sith, and those feelings presumably didn't change once he required hate and rage as his sources of power. But it goes beyond that, from the ranks of the Imperial Navy he killed to the way the heroes seemed to slip through the Vader's fingers every time.
He Destroyed the Imperial Hierarchy
Vader was infamous for Force choking those beneath him when they angered or insulted him. The threat was omnipresent, from Connic in Rogue One: A Star Wars Story to Admiral Motti in Star Wars: Episode IV – A New Hope, although Grand Moff Tarkin kept a tight rein on it and prevented any fatalities. Once Tarkin died, however, Vader went wild, Force choking both Admiral Ozzel and Captain Needa to death on more or less momentary whims. That kind of wholesale destruction of senior staff can wreak absolute havoc on a military’s chain of command, confounding logistics and leaving the Imperial Fleet unable to react when faced with a better-organized foe.
Indeed, Vader caused almost as much damage when he chose not to Force choke someone as when he did. He famously declined to kill Admiral Piett at the end of Star Wars: Episode V – The Empire Strikes Back, a man who not only allowed Luke and Leia to escape Cloud City, but subsequently said little, tried nothing and died ignominiously at the Battle of Endor aboard the Executor. It’s hard to imagine Ozzel or Needa doing worse in such circumstances.
He Let the Rebels Escape Time and Again
Beyond the havoc he wreaked in the Imperial war machine, Vader repeatedly made decisions that let key Rebel heroes escape. The first time came in A New Hope, when he insisted on facing Obi-Wan Kenobi alone rather than sounding an alert and locking down the entire Death Star. Furthermore, he had a homing beacon placed aboard the Millennium Falcon over Tarkin’s open concerns, rather than just sending the whole fleet to retrieve them. As a result, the Rebels retrieved the plans and destroyed the Death Star at the Battle of Yavin. That trend continued in Star Wars: Episode VI – Return of the Jedi when he allowed the shuttle to land on Endor despite knowing that Luke was onboard. Instead of being taken into custody immediately, the Rebels landed on the moon, made contact with the Ewoks and dealt the Empire another crippling defeat.
Anakin’s appearance as a Force ghost at the end of Jedi has been the subject of much debate among the fans, but if he was serving a greater good by secretly sabotaging the Empire at every turn, it might help explain his postmortem forgiveness more readily. Regardless, the Emperor let Vader feed his hate, which made him powerful at a terrible, and ultimately fatal, cost to the Empire he was supposed to serve.