renegade
A person who leaves their group and rebels against it.
A renegade is someone who rejects the rules or beliefs of the group they once belonged to and strikes out on their own path. The word carries a sense of rebellion and independence, but also of betrayal or abandonment.
In history, a renegade soldier might desert their army to join the opposing side, or a renegade member of a club might refuse to follow the group's decisions. Sometimes renegades leave because they genuinely believe their old group was wrong. Other times, they simply want freedom from any rules at all.
The word can be used as an adjective too: a renegade scientist might pursue research that breaks established methods, and a renegade basketball player might ignore the coach's plays to do things their own way.
Calling someone a renegade depends on your perspective. To those who valued the old group's loyalty and rules, a renegade looks like a traitor. To those who admire independence and courage, the same person might look like a brave pioneer. In Western movies, renegade outlaws break society's laws, but they may follow their own strict code of honor.
The key element is always the break from the original group. You can't be a renegade without first belonging to something, then choosing to walk away.