Absolutely Evil

[alatar]

While listening to FotR this morning, I walked with the Hobbits as the entered the Old Forest. The trees therein, as we know, did not take too kindly to our adventurers and so did what they could to hinder the Hobbits' steps. Eventually the trees steer the four down to the Withywindle where Old Man Willow lay in wait.

In Middle Earth we have the forces for good (Gandalf, Galadriel, Frodo, etc), the Enemy (Morgoth, Sauron, the Witch-King, the Wights, etc) and here the trees with Old Man Willow presumably as their leader or at least the first among them. Which side is the willow on? As Treebeard will state later in TTT, he and the trees aren't on anyone's side as no one is wholly on their side. Is there a third side, neither good nor evil, but benign until threatened, then attacking like a cornered animal?

Or did some taint enter the trees from the Void, siding them with the Enemy?