There are countless synonyms for the phrase 'be bad.' Some of the most common include terms such as misbehave, act out, misconduct, or behave inappropriately. These words can apply to a range of different behaviors, from failing to follow established rules to engaging in more serious criminal or unethical activities. Other synonyms that may be used according to context include words such as break the rules, act up, violate norms, or even defy authority. Ultimately, the most appropriate synonym will depend on the specific situation and the gravity of the behavior in question. Regardless of the word chosen, however, the message remains the same: there are certain behaviors that are simply not acceptable, and individuals who engage in them must be held accountable.