I want to be sure I understand this, so I am turning to the opinion of the ArM groupmind.
According to ROP: The Infernal, an Infernal aura drops in power when it is overwhelmed by a greater aura, slowly, over a period of years. Otherwise the Internal aura persists.
An Infernal aura of sufficient potency Warps the place it covers and the people who live there in steady and corrupting way.
Case in point: The city of Beziers was the site of a massacre of innocents, and has an appropriate Infernal aura of, usually, 7 over much or most of the city, or at least the location of the massacre (which I understand was much of the city). The cathedral and churches and a few other spots probably have overwhelming auras, but most of the city is Infernally influenced, which leads to sinful behavior and feeds a cycle perpetuating the aura.
Furthermore, the aura is high enough to warp the city and people, so the place is actually getting worse and absent some kind of intervention will continue.
Have I missed something?