Albert Barnes Commentary Acts 22:29

"They then that were about to examine him straightway departed from him: and the chief captain also was afraid when he knew that he was a Roman, and because he had bound him." — Acts 22:29 (ASV)

Then straightway. This means immediately. They saw that by scourging him they would have violated Roman law and exposed themselves to its penalty.

Which should have examined him. This refers to those who were about to torture him by scourging him (Acts 22:24).

Because he had bound him. This was done in preparation for scourging him. The act of binding a Roman citizen with such an intent, while he was untried and uncondemned, was unlawful.

Prisoners who were to be scourged were usually bound by the Romans to a pillar or post, and a similar custom prevailed among the Jews.

That it was unlawful to bind a Roman citizen in this manner while he was uncondemned is evident from an express declaration in Cicero (against Verres): "It is a heinous sin to bind a Roman citizen; it is wickedness to beat him; it is next to parricide to kill him; and what shall I say to crucify him?"