John Calvin Commentary Acts 16:38

"And the sergeants reported these words unto the magistrates: and they feared when they heard that they were Romans;" — Acts 16:38 (ASV)

They were afraid, because they were Romans. They are not in the least moved by the other point—that they had handled innocent people cruelly and indiscriminately—even though that was the greater reproach. But because they did not fear that anyone would punish them, they were not moved by God’s judgment.

This is why they carelessly ignore the charge concerning the injury they had committed; their only fear is of the Roman officials and of being beheaded for violating a citizen's liberty. They knew that this offense meant death if any of the chief governors (prefects) committed it; what then would become of the officials of one free city?

Such is the fear of the wicked: because they have a stupefied conscience before God, they flatter themselves in all sins for a long time, until human punishment hangs over their heads.