Ok, RoP:I has some nasty extra effects for Infernal Auras - e.g a Malevolent Aura. Among others the aura rating adds to any damage roll there, and all physical acvitities recieve extra Botch dice equal to aura.
But should the area of infernal aura also contain an Regio - e.g. an Abyssal Regio - does these aura effects also apply here?
The inside of the regio does have an aura, but is this a 'vanilla' aura rather then the kind outise the regio? In this case a Malevolent aura?
It could certainly go either way, but I don't see The Infernal as something that ever gets "easier".
If I were in a saga where that happened, where a "malevolent" infernal aura led to a "vanilla" infernal regio, I'd immediately suspect some demonic deception, and worse things to come.
And it's that very situation that does not sit well with me. However in an Abyssal regio there is not really 'vanilla' as the Aura subtracts from all characteristics and you roll to avoid going mad over time (well, temporary flaw that is). But the aura itself does not do what it does in a Malevolent Aura (adds to damge, the # of botch dice, plus makes old wounds caused by demons reopen).
But is it worse or the same overall to go from the Malevolent aura to the AByssal regio?
Do the Malevolent sideeffects also apply insode the regio as standard?
Or can they, if you are an evil SG?
Of course they can.
We start with an Aura, with a Regio (hidden) within it.
The Aura becomes "tinged". (EDIT - proper IG term is "tarnished", from ROP:Infernal p 13 etc. - my bad. Fixing below...)
Then question then is: Has it become (tinged) tarnished simply by accident, or by infernal design? Did the random sins of man (tinge) tarnish the aura, or did the presence/deeper nature of the Regio allow/invite demons to concentrate their interest on the souls found there, and thus the Regio became the reason, not the result?
(Could the Regio even have been created by great sin? I don't see why not...)
So, it can work either way. And it probably should not work the same way twice, or at least not "because that's how it always works".
I would say it should not get "better".
Whether it gets worse or not in the first circle of the Regio is up to you. But Regio can have multiple levels - I'd certainly expect it to get worse if you go deeper, tho' "worse" could mean many things.
I just gotta find the right balance. It might be too easy if the malevolence doesn't also affect the regio. But it might be too hard if it does!
Depends on the aura rating overall I think. A lvl 3 malevolent aura is mean but not crippling, some extra damage and bad botches. Adding that to a lvl 3 abyssal regio means all characteristics are -3 as well, and adding that with the increased risk of botches as well as higher damage makes this a nasty place.
But doing that same thing with a lvl 7 aura...ouch! But noby ever said poking around in hell's antechambers was easy, comfortable nor funny.
I think the my demons would not let the Malevolent aura go into the regio. I would make them "pay" to remove it, though, since that state would help them a lot:
If the humans feel that it is better to be in the regio, they will seek to get there to get out of the malevolent aura. In the regio I (as a demon) would have more power (stronger aura) and use that for subtler effects. The "relief" of getting into the regio and out of the malevolent aura should soften the targets for my subtler attacks on their souls 
Normally though I would say that a taint (or other flavoring of other types of aura) would start on one level of a regio (normally at the mundane or the highest level, in my world) and would slowly spread to all of it unless conscious effort from either natives or knowledgeable mundanes is applied to avoid it.
(I like some of that! Not perfectly "by the book", but I'll be the first to say that Demons should not be!) 
For those without RoP:Infernal (or just "not at hand"), and for my own reference/refresher, let's nail down the terms in order of unpleasantness:
Infernal Aura: As the dominant aura in an area, it modifies all supernatural powers and spellcasting rolls, and causes any open wounds to become infected, acting as a penalty to healing rate - things from the core book. That's all, but that's often enough.
Optionally, any (permanent) Infernal Aura can also be a...
Tarnished Aura: has a "sin" personality trait rating, that modifies all character personality traits in that direction.
Corrupted Aura: Permanently tarnished, and of at least Aura 3. 3 broad categories:[list]o Debauched - encourages/embodies sins of self-indulgence. While there characters want to sin, area is hard to leave, and strongly tempts characters to return.
o Malevolent - encourages/embodies sins of violence. embody cruelty. Jagged, twisted, poisonous, lifeless places. Aura is added to Damage, adds to botch dice, botches tend to be painful injuries, inhabitants (who survive) become corrupted.
o Perfidious - encourages/embodies sins of betrayal. Lies are more believable and harder to spot, can affect Confidence points.
Infernal Regione: most abilties related to "God" are voided. 2 types: o Tartaran: places of hellfire. Hot. Hotter than that.
o Abyssal: "Godforsaken" - life, inspiration, reason - all are diminished. Literally body- and mind-numbing.[/list:u]Readers should note that only Malevolent Auras have are of the "extra damage/extra botch" type we're talking about in this thread. (And there are more penalties, all mechanics are in RoP:Infernal, recommended if the above is even a little bit attractive.)
This, above all...
But in finding that balance, you don't want it to seem too heavy handed. You want it to either "make sense" within your game, or to appear an obvious and intentional "mystery" - not "Gee, that's pretty arbitrary...".
Well, also consider those penalties to any "normal life" at just Level 3. If an area were that bad - regular accidents with inexplicably horrible wounds, constant problems and general anti-social behaviour - no one would live there, or if some few did it would be a real "Hell's kitchen" sort of environment - run-down, squalid, animalistic and violent, unwanted with only the most unwanted of society there. Bodies buried in walls (when at all), newborns abandoned to the feral cats and dogs - that sort of thing.
So, the moment the characters enter a Malevolent Aura, they know that the shit's gonna hit the fan - they just know it, or (sorry to call a spade a spade) are just too ignorant/stupid to run this adventure hook. They just are. So make sure that, IC (if not OOC), either their characters know what they're stepping in to or are given strong enough hints that they should find out before proceeding!
Now...
None of the Regiones (as described in the book) care about "sin" - sin only relates to man's relation with God, and once you're in an Infernal Regione you're past "God" and all that. By the book, the Tarnish does not extend into the Regio. I think the idea is that a horrible -7 Malevolent Aura would give way to nothing less than a Level 7 Regione, and the penalties there are bad enough, if in a different way.
However, that said, there's nothing to prevent it from happening if it makes sense to you. Regiones do mimic elements of the "real world" beneath them, so beyond the physical layout you could apply some of those same penalties in one form or another.
As you said - "balance". For your story and your "heroes", and for your players and their level of play/enjoyment. If you're at all worried that they may not be up for it, tone it down, and/or just keep hammering them at every stage with how bad things are and convince them that this is just "reconnaissance" and they should come back when they're ready - if ever. If they're hard-core, both as Players and Characters, and they think they can handle it and you think they're right - go for it. 
[i](But you have to know your players. Long before RoP:Infernal came out, I ran Festival of the Damned as one of my very first AM adventures - but didn't know these players as well as I thought. They refused to back down or give up in the face of "clearly" superior Demons - they were used to "winning", each and every adventure. When they didn't, and it was almost a TPK, we knew each other better, and had some "game theory" to discuss over more than a few beers. 
The saga continued, and better after that, if with a bit more caution on both sides of the table.)[/i]
I was just wondering what was RAW, I don't think it is perfectly clear. But upon reflection it makes sense that the Regios are totally different than whatever sin tarnishes the aura outside it. A lvl 7 Abyssal Regio is nasty enough by itself, since all characteristics are reduced by 7 while inside. Ouch!