The phrase "be traitorous" generally denotes being disloyal, treacherous, or deceitful. Some synonyms that can be used in place of "be traitorous" include phrases like "betray confidence," "be false," "go behind one's back," "sell out," "stab in the back," or "double-cross." All of these phrases convey a sense of deception or betrayal, whether it's against a friend, colleague, or even a whole country. Some additional options may include more specific phrases, such as "turncoat," "apostate," or "defector," which can denote someone who switches sides or allegiances for personal gain or spite. Overall, the choice of synonym will depend on the context and intended emphasis of the underlying meaning.