How had the ethos of the political elite changed since the epoch of Scipio Amelianus through to the end of Augustus’ reign?

Following the destruction of Carthage in the Third Punic War, Romans were finally free from major external threats. This uncontested supremacy led to a turning away from a defensive mindset to one that focused increasingly on luxury, pleasure, and personal ambition. This failure of the elites to continue with the ideals of the Republic and instead focus on avaritia (avarice) trickled down to the lower classes, who were increasingly catered to with bread and circuses to ensure amusement and, thus, stability.
With ultimate power came ambitious individuals ready to climb the ranks and assume the glory and spoils that came with it, most notably Julius Caesar, who was instrumental in bringing about the end of the Roman Republic. But glory and the spoils of war were not the only things that fueled the eventual rise of the Roman Empire. Under Augustus, the notion of civilizing barbarian peoples also served a more practical purpose: with civility came Pax Romana (Roman Peace) as well as the expansion of markets that could fill the coffers of the empire.
We are left with a complex set of reasons for the changing in the ethos of the political elite. The destruction of an ancient foe (Carthage) certainly allowed for relaxation, greed, and the chance for ambitious individuals to assume power—even if that power was ultimately self-destructive. However, the needs of an ever-expanding empire also required a shift towards policies that would ensure the continuation of Rome, even if those policies conflicted with the earlier ideals of the pre-imperial Roman Republic.
The Roman senator and historian, Tacitus, perhaps summed up these changes best when he wrote, "From time immemorial, man has had an instinctive love of power. With the growth of our empire, this instinct has become a dominant and uncontrollable force."