He was cloned near the end of the war and assigned to the 501st Legion in time to take down the Jedi Temple but was injured in battle while attempting to take down a Jedi Master during the purge with a head injury. This head injury somehow allowed to go against his innate programing to be loyal to the Empire. While he respects the Empire its rigidity to military discipline he does not like the way his fellow clone brothers were used as pretty much slave labor. He also has no love (or hate) for Jedi, he thinks most of them made poor Generals and less of his brothers would have died if somebody who was trained for war was in charge. So during the time he was recovering in the hospital, he just got up and walked out, somehow managed to get off planet and into edges of the galaxy. He scrounged up some credits, had a fake name set up and became a farmer. This all changed when he heard about the destruction of Alderaan, he instantly left his farm and joined up with the Rebels.