They can magically attach bad karma sins to someone to send them to Hell? And also raid Hell to rescue people? And they can bring the dead to life some other way - that sounds far too abuseable, Tanya has no idea if it's real or what equilibrium it leads to and doesn't really want to admit this publicly so she'll ask Belmarniss later. She is going to have so many questions.
(And they don't have reincarnations by default? Apparently? Tanya isn't sure Earth does either, she has no real evidence it's routine and Being X is a lying liar.)
...
Anyway. Demons are the damned who spent too long in the Abyss, which is an inherently corrupting environment. Putting all the lawbreakers in one place with no supervision will reliably do that even on Earth! In other words, it's an eternal super-prison with no guards or parole except for raiding adventurers, and the older inmates lord it over the newcomers. (Infinite punishment for finite crimes is wrong but religion always does that.)
Now the inmates have managed to escape / invade the mortal world, by opening a permanent portal instead of using expensive one-off spells (because they are not hopeless at logistics) (unlike Hell, apparently) and/or because they have better technology. The locals naturally don't want their executed criminals to come back. The ex-criminals naturally want to get revenge, commit more crimes, et cetera. This war was probably inevitable once someone figured out how to open a portal.
This does sound like a justified, defensive war. One all the nations of Golarion have an equal interest in, because the demons come from all nations and likely won't settle for conquering just one. That's assuming she takes everything she's told at face value. It's also very easy to interpret this as wartime propaganda, backed by some churches declaring that killing those people isn't evil after all. (Tanya isn't even talking to a priest of god the Judge, assuming he has priests.)
If it's not propaganda, then Tanya obviously hates this system but at least she's on this side of it - oh. "Can you tell me about Hell? Is it - symmetrical, with people sent there becoming, uh, devils because it's a place that - bends people towards evil and lawfulness?" Being 'bent' (incentivized?) to become more lawful sounds like a good trait for a place to have, but not if the laws are evil.