The Romans are complaining now about the awful things that have happened to them when the barbarians sacked the city, but they seem to have forgotten that their own state was founded upon exactly the same atrocities. Nor can we say "that was just the times of the monarchy, under the republic things were better", since the history of the Roman Republic was just as unjust and awful. So much so that "I cannot now mention all the shameful and iniquitous acts with which Rome was agitated," and this is a 1200 page book. Augustine has time to mention everything!
Even the pagans themselves (in the person of the historian Sallust) admit that the best moments of the Republic weren't so much driven by virtue as they were by fear. That is, the times it appeared that the Romans were living a virtuous existence were not actually virtuous, since they were driven by fear of destruction rather than the proper motives (though at this point Augustine has not yet told us what those proper motives are). The point is, even when the Romans were good they were bad, and now they're not even good. So it makes no sense to blame Christians or Christ for the current dreadful state of Rome; it has really always been dreadful. On the other hand, the most that Christ has done is be the one "who teaches life-giving truth, and forbids us to worship false and deceitful gods, and who, abominating and condemning with his divine authority those wicked and hurtful lusts of men, gradually withdraws His own people from a world that is corrupted by these vices, and is falling into ruins, to make of them an eternal city, whose glory rests not on the acclamations of vanity, but on the judgment of truth."