The Roman Emperors are a good example. They were evil rulers, like virtually all temporal rulers of that time or indeed of any time, but they were legitimate rulers over those territories that acknowledged their rule.
The fact that a ruler is a sinner doesn’t disqualify his reign. If it did there would be no legitimate secular authority in the world. Now, sometimes the State may order things it has no right to order, like telling people to do something sinful. In that case one would have no obligation to obey, but indeed would have an obligation to refuse to obey, because civil authority is subsidiary to God’s authority and civil law to natural law.
