Despite our tendency to cover the past in a golden hue, the reality is that even the leadership of the city of man is fallen and comes to a bad end. Romulus himself was torn apart in the Senate (the legend that he became a god is a verifiable falsehood), while countless other Roman kings met their deaths often even at the hands of their own family.
In fact, the kings in Roman history who seem to have the most success (and so be the most blessed by the gods) are often the worst ones morally--the parricides, the tyrants, the bloodthirsty and cruel. As a consequence, we either have to say that the gods value such behavior, or that they are no gods at all
The Republican period of Rome is no better. Having expelled the kings, the Republican leadership immediately begins killing themselves off in a grab for power. Brutus himself, the George Washington to Tarquin's George III, killed his own sons, as well as expelling his co-ruler merely because he had the same name as the former king. If this is the golden era of Rome, it is hardly evidence that Christianity has led to moral decline!