Why is Jem considered a "traitor" in To Kill a Mockingbird?
When Dill runs away from home and is discovered hiding under Scout's bed, Jem is the one who decides to do the right thing and tell Atticus. Because Jem seemed to "side" with the adults instead of with Dill and Scout, Scout labeled him a traitor. This is an example of Jem's growing maturity in that he does the right thing and not the popular thing with his sister and friend.
Dill runs away from home one summer, and Jem and Scout find him hiding under the bed. Jem insists that they tell Atticus, but Scout feels Jem has broken a law of childhood by betraying Dill. This shows how Jem is maturing because he understands how worried Jem's parents and Aunt Rachel will be. Scout and Dill see Jem as ratting Dill out to the adults, taking their side over Dill.