Star Wars: The Clone Wars, his career began in the days of the Galactic Republic, and he swiftly drew the attention of Chancellor Palpatine. Tarkin was perfectly suited to the new order - and he rose through the ranks of the Empire at prodigious speed.
Although Grand Moff Tarkin was no Sith acolyte, he had a disturbingly similar philosophy. The old Expanded Universe introduced the concept of the so-called "Tarkin Doctrine," with Wilhuff Tarkin teaching the entire Empire it should be established on the principle of ruling by fear. This idea has been carried forward into the modern Star Wars canon, and indeed it has been expanded upon considerably. In Tarkin's view, the vision of a galaxy at peace required a strong military and a cowed populace - and the Death Star was to be the greatest symbol of that military, terrifying all worlds into bowing before the might of the Empire. When the Death Star was finally complete, Tarkin unleashed it upon the peaceful planet Alderaan - but it's interesting to note he didn't require permission from the Emperor.
The latest Star Wars book, Alexander Freed's Victory's Price, suggests the reason he didn't need permission; it's likely he already had it. It seems that, while all attention was focused on the Death Star, Palpatine had constructed another superweapon - one that was far more insidious. He had transformed the Empire into the greatest surveillance state the galaxy had ever seen, and he had constructed a massive supercomputer on Coruscant to process all this data. This supercomputer recorded the actions of every Imperial citizen in the galaxy, identifying the corruption Palpatine was ensuring seeped down through the Empire. No doubt the Emperor swiftly identified planets where his dark vision was not being realized, societies and civilizations that were resisting him. Alderaan would have been top of the list - and consequently it would have been a major target.
Ironically, the destruction of Alderaan backfired. As evil as the Empire had been, the citizens of the Core Worlds had been able to blind themselves to its atrocities because they were all conducted on distant worlds nobody had heard of. The destruction of a Core World planet was a shocking act, unparalleled in galactic history, and it encouraged rebellion. Making matters worse, the Death Star was swiftly destroyed by Luke Skywalker in the Battle of Yavin, and Grand Moff Tarkin with it.
It also served to transform Princess Leia Organa into one of the most powerful symbols of the Rebellion. She had paid the ultimate price, losing her entire homeworld and the Organa family, and yet she continued to oppose the Empire. The story of Star Wars is essentially the story of how the dark side will ultimately defeat itself, because evil concepts like the Tarkin Doctrine may be in line with the way of the Sith - but they are also self-defeating.
