The Cabal we fight on mars are not actually part of the Empire. They're the remnants of a rebellion that was lead by Valus Trau'ug(you fight him in PoE along with his Val.) Trau'ug wanted to attack the Reef during the Reef Wars while the Awoken and Fallen were busy with each other. Cabal command refused is request and ordered him to withdraw from the Reef area. Trau'ug blatanlty refused the order, killing any under his command who did not support him. The Awoken met his advance with a readiness that Trau'ug did not expect. He was quickly defeated and captured. In his absence Taur'uc took up the title of Valus in support of his captured commander.