Background
Former Imperial transport pilot, he wasnt able to join the ranks of TIE pilots within the imperial navy, but scored well enough to earn a place as a shuttle and transport pilot for ranking officers. His loyalty was never in doubt as he was never in a position to question orders.
He became disillusioned with the empire when he witnessed first hand his assigned star destroyer performing an orbital bombardment to eradicate a rebelling city that had practically no ability to defend themselves. He had interacted with the locals, decent people that felt the empire was taking from them more than they could provide for themselves.
While this wasn't a singular event that made him leave the empire, it was the final straw in a long series of missteps the empire had taken.
Motivation
Honor