when should you use 배신 and 배반 ?
배신 is used more often than 배반, especially in day to day conversations if needed, but both of them convey the meaning of betryal.
In my opinion, 배신 is often used when someone betrays someone else.
- 그는 친구를 배신하고 약속을 지키지 않았다.
- 내 친구 광수는 일평생 배신한번 해본적 없는 믿음직한 친구다.
On the other hand, 배반 carries a bit more subtle differences in meaning where you can use it as someone betrays a group/organization/country AND also turns against them. It also somewhat depicts stronger betrayal than 배신.
- 그는 나라를 배반하고 적에게 비밀정보를 넘겨주었다.
- 그녀는 자신의 가족을 구하기위해 조국을 배반할수밖에 없었다.
But because they still hold similar meanings, you can probably use them interchangeably and readers will likely understand.