Background
One among the millions of Jango Fett clones that served in the Grand Army of the Republic, Bastille "Bast" was born under the number CC-363; a special designation for clone commandos bred and trained to undertake the most perilous missions available. He was assigned to serve under Jedi General Qhoren Visha, operating as a covert infiltration team tasked with eliminating high value Separatist targets as well as uncovering information and destroy key facilities. Among the Nu Squadron, Bastille was known as an anchor, capable of holding strategic points by himself for hours, potentially days...
He had survived the war, though was stricken with guilt and severe post traumatic stress; when Order 66 came, he was on deployment deep beneath enemy lines in one of the final separatist strongholds in the outer rim; his brothers were spread out around the planet, eliminating their targets but Bastille had been elected to protect General Visha... Alone.
Bastille and Visha were good friends, thus it came as a shock to the Jedi when he awoke to find Bastille's gun pointed at his temples; the shock didn't last long, his life snuffed out alongside thousands of other jedi, all in the span of a few minutes.
Killing one's own friend is enough to break most men, especially when its so far out of your own control. Following the end of the Clone Wars, the Grand Army was decommissioned and the Empire that came in the wake of the republic hardly cared for the clones that had fought for their foundation. Bastille's mind was slowly broken down as he began to buckle and break beneath the weight of his guilt, it wasn't until a twi'lek named Koyi Ameulla discovered the biochip in his mind that he began to fight back-- For his purpose had been, all along, to destroy that which he was protecting.
Someone had to pay.
Motivation
Vengeance
After order 66 and the decommission of the Grand Army of the Republic, Bastille was left broken; he had become close friends with his general and being forced to turn his gun against him broke down his mind. Over the course of the following years, Bastille became more and more erratic, secluding himself from the galaxy at large until he managed to discover the biochip implanted in his brain, following a risky surgery the clone emerged in the galaxy once again. Not to sulk, or to complain, but to destroy the Empire that had built itself upon the corpses of his brothers and his closest comrades.