General:
* Renegade
* Turncoat
* Traitor (implies betrayal)
* Runaway
* Defector (especially for military personnel switching sides)
* Absconder (implies fleeing to avoid punishment)
* Fugitive
More specific:
* Draft dodger (for someone who avoids military service)
* AWOL (Absent Without Leave, specifically for military personnel)
* Deserter (this word is quite strong and implies a betrayal of duty)
The best synonym will depend on the specific situation and the nuances of the context.