In some previous posts I've made attempts to un-hippie D&D's portrayal of Druids, forests, and Rangers. From my own, admittedly limited, research I've found that the Druid especially has done a 180 since its introduction in Greyhawk. Originally a monster, the implication seems to have been that Druids were the sorts of priests that Romans found terrifying. They may be Neutral, but if one takes the stark Law vs. Chaos element which seems to be present in B2: Keep on the Borderlands, then one can see that Neutrality is a rather dangerous prospect if you're on the side of Law.
This is not to say that Druids are evil, just that they're harmful to Law.
Another factor in my attempt to turn the forest into something more sinister is the Medieval treatment of forests. I should note this does not apply to the Late Middle Ages, whose forests weren't really forests at all but simply rolling lands of farmsteads and hunting preserves. Rather I'm talking about the way it tends to be portrayed in Christian literature in the Early and High Middle Ages. In those works the forest is, to re use my title, where the Devil lives. Or rather Devils, as the Medieval concept of Satan is very loose and is often composed of numerous demons who swarm in the Infernal Pit.
Paganus which is often translated today as pagan, actually means country dweller or -- more comically -- bumpkin. Heathen means an inhabitant of the heath. The wilderness's association with pagan religions turned into an association with the Devil in Christian writing. If that's where the pagans themselves live, that must also be where the demons who beguile them make their nightmare-homes.
The final factor in my attempt to turn the forest into something scar is Clark Ashton Smith's Averoigne. While I love Zothique and Hyborea, Averoigne's association with France and the Middle Ages makes it perfect inspiration for my purposes. To give you some idea of what I'm drawing from that cycle, I'll close with this quote.
It was not likely that they would be seen or interrupted; for the gnarled and immemorial wood possessed an ill repute among the peasantry. Somewhere in this wood there was the ruinous and haunted Chateau des Faussesflammes; and, also, there was a double tomb, within which the Sieur Hugh du Malinbois and his chatelaine, who were notorious for sorcery in their time, had lain unconsecrated for more than two hundred years. Of these, and their phantoms, there were grisly tales; and there were stories of loup-garous and goblins, of fays and devils and vampires that infested Averoigne. But to these tales Gerard had given little heed, considering it improbable that such creatures would fare abroad in open daylight. -- Clark Ashton Smith, "A Rendezvous in Averoigne"