Class traitor is a term used mostly in socialist discourse to refer to a member of the proletariat class who works directly or indirectly against their class interest, or what is against their economic benefit as opposed to that of the bourgeoisie. It applies particularly to soldiers, police officers, workers who refuse to respect picket lines during a strike, and basically anyone paid a wage who actively facilitates the status quo. According to Barbara Ehrenreich: "Class treason is an option at all socioeconomic levels: from the blue-collar man who becomes a security guard employed to harass striking workers, to the heirs of capitalist fortunes who become donors to left-wing causes"
In Russia before and during the Russian Revolution, the Bolsheviks and other socialist revolutionary organizations used it to describe the Czarist Army and basically any working class citizen who directly opposed the "Dictatorship of the proletariat". The term was later extended to include the Menshevik Russians and other supposedly counter-revolutionary socialist organizations under Stalin.
The motives behind becoming a traitor to one's class can include the necessity of survival (taking up whatever wage is available), the belief that the person is of a higher class and so has political views that work against the working class, the pressure of conformity, or a rejection of the view that society is divided up into antagonistic classes.