The faulty stands on his guard.

A person who has done wrong is especially watchful, expecting suspicion, exposure, or consequences. Here, “faulty” means a person at fault or guilty, rather than merely someone who makes mistakes; their guarded manner may arise from awareness of their own fault.

The saying can also caution against treating vigilance as proof of guilt. It describes defensive alertness often associated with wrongdoing and suggests a tendency, not a reliable test: a wary person does not necessarily have something to hide.

Use it in reflective discussion of secrecy, accusation, or defensive conduct, while avoiding claims that guarded behavior proves guilt. The phrasing is formal and old-fashioned, so it suits literary or historical-toned writing better than casual contemporary speech.