right why's he done that then

Rue keeps on mistaking Na-in, it's true that he only wanted to serve him and only him.. He thought since Gwen gained Nain's favor (since he hated Gwen at first) Rue thought being a Leshah is better for his devotion (since Leshah's are created to literally amuse and be used by Na-in)

In conclusion, Right heart, wrong actions by Rue