Well, here's more of my crazy theory that I developed last night: what if the rapid industrialization of the past decades has thrown the human and the spirit world out of balance? Remember how angry Hei Bai was about the forest fire that destroyed his grove? Presumably the kind of mining and logging it would take to build a place like Republic City has never been seen in their world before -- and the spirits may see the benders as being at fault, since it seems like a lot of the tech in their world is based directly or indirectly on bending powers, not our science.

So spirit intervention here might be more like "justifiably pissed off" than "evil incarnate". (And it seems like spirits don't handle anger very productively, to say the least.) Koh has intervened in our world before, when he stole Ummi because he thought Kuruk was a crappy Avatar. And I need to watch the ep again, but didn't Amon say that the fire bender "stole my family from me, and even stole my face"? That seems like a weird way for the writers to phrase things, unless they're foreshadowing the Face-Stealer.

Just a theory, though!