Why does Lonnie betray Clem

“Kneel to the Rising Sun” tells the story of Lonnie Newsome, a white sharecropper who works on the property of Arch Gunnard. Lonnie and his family are underfed and slowly starving, and when the story begins, Lonnie approaches Arch to ask for more rations. Arch, who presumably knows why Lonnie is there, cuts off the tail of Lonnie’s dog before eventually promising to give him some more food in the morning. Lonnie is unable to work up the courage to stop him.
Clem Henry, an African American sharecropper, witnesses the exchange, and Lonnie is aware of Clem’s anger. This opening sequence foreshadows what is to come in the story’s climax. When Lonnie’s father is found dead, Clem confronts Arch, and the two fight. Clem runs off to hide, but Lonnie betrays him by telling Arch where he is.
Lonnie is both envious and afraid of Clem, and these conflicting emotions manifest in his betrayal:

Lonnie looked at Clem fearfully. He knew Clem was right, but he was scared to hear a Negro say anything like that about a white man.

Lonnie is envious of Clem because Clem has the courage to speak up to Arch. Lonnie knows that he is abused by Arch, but he constantly justifies his own behavior. Lonnie knows how much he and his family are struggling, but he lets race dictate his own actions:

He was a white man, and to save his life he could not stand to think of turning against Arch, no matter what happened.

Even though Clem is right, in the final instance Lonnie reduces the situation to a question of race. Clem has more courage than Lonnie, and because Clem is African American, this becomes a source of hatred for Lonnie. Ultimately, this motivates him to betray Clem.