Other forms: defectors
A person who quits something, despite a perceived duty or obligation, is a defector. If you abandon the Boston Red Sox to root for the New York Yankees instead, your fellow Bostonians will consider you a defector.
In politics, defectors abandon their home countries and claim allegiance to another. A person can only be a defector if the first state forbids such a change and considers it illegal or illegitimate, as the government of North Korea does when one of its citizens leaves the country. Abandoning a military post also makes someone a defector. In your life, you're more likely to be considered a defector (by some) if you change political parties or stop being a vegetarian.