Harkov

Admiral Harkov served in the Imperial Navy until about 3 years ABY. He commanded the Victory-class Star Destroyer Protector and had a small fleet under his command. One of his last assignments as an Imperial Admiral was to pacify the Sepan system; which was in the grips of a civil war between two factions; the Ripoblous and the Dimok. After the Imperials inflicted heavy losses on both sides; the two factions sued for peace. However, all was not as it seemed. Harkov had pilfered weapons and equipment from the Empire, and sold them to groups including the Rebel Alliance and was probably selling them to the Dimok as well.

Shortly before the Sepan assignment; Harkov approached leaders of the Rebel Alliance and announced his intention to defect, and promised that his fleet would defect with him. Shortly after the end of the Sepan Civil War, Harkov carried out his plan, and defected. He was eventually hunted down and captured. He was brought before Darth Vader and after a short interogation, he was killed by the Dark Lord.

Source
Star Wars: TIE Fighter PC game.