It feels as though we have been building up to this discussion for some time now, always dancing around the topic of good and evil but rarely exploring what they might actually mean. The dichotomy of good and evil is ever present and often sharply contrasted in all of Tolkien's work; despite some small grey area, there are clearly evil forces and clearly good forces.
One of the key questions relating to evil in The Lord of the Rings is of course the question of just what is the One Ring. In our reading and in class it has been described variably as pure power, as an entity with a will of its own, or as a corruptor of beings, among other things. There has been much talk of the Ring binding the wills of the people of Middle Earth to its own will, or to Sauron's will. But the process of doing so is far from automatic. Frodo, of course, gradually becomes more susceptible to the Ring's influence the longer he carries it, from the first few times he puts it on to the episode at Minas Morgul all the way to Mount Doom where he finally succumbs and chooses to preserve the Ring. Boromir travels with the Fellowship for some time before he attempts to take the Ring from Frodo. The Nine, while not in direct possession of the One Ring, are never the less bound to it over time through their own rings, turning into wraiths before they are even fully aware of the transformation.
One theory of evil proposed it that evil is simply the absence of good, that men (or elves, or dwarves, etc) in their natural state without the constraints of morality will turn to actions that we would consider to be evil. Some suggested that the Ring removes these constraints by giving power to its bearer ("absolute power corrupts absolutely") but there may be more to it than this. Perhaps the Ring's primary power is that it erodes wills, stripping its bearers of power rather than giving them power. What better way to bind wills to its own if said wills are eroded?
However, there is the problem of Gandalf, who claims that with the Ring he would become remarkably more powerful than he is. Even more importantly, he believed that he would exert a form of domination over the people of Middle Earth. How then, do we reconcile this with the idea that the Ring primarily weaken those who would hold it?
In the LeGuin reading, the Hans Christian Anderson story about the man and his shadow was discussed. The shadow, the part of a human unbridled by societal and moral restriction, is compared to the wraiths from The Lord of the Rings due in large part to both of them being incomplete in some way. The Nazgul, as well as Gollum, have lost some essential part of what made them living beings with souls before. In the case of men, what made them men was their free will and ability to shape their own destiny. This is what was taken away from the Nazgul. In other words, the Ring strips those it affects of something fundamental to their being. Returning to the case of Gandalf, what he would have lost in seeking to dominate Middle Earth would be his passive wisdom and also his mandate and mission as one of the Maiar to assist without exerting power over people. It would have been a violation not only of his orders but who he was, given his purity and benevolence. Boromir wanted the Ring to defend Gondor from the forces of Sauron, but how long would it have been before his actions became motivated by merely inflicting vengeance and pain on those dark forces with little regard for his own people.
So the Ring does indeed give power to its bearers, but it is an empty power, since they lose themselves before they can use that power to enact things which they might have seen as just. Not all of the Ring's bearers become such obvious slaves as Gollum, but even with a facade of power, they all become slaves none the less. For the very fact that the Ring is able to change people so fundamentally means they are enslaved to it, whether it appears that way or not. Sauron himself may be the only one to not lose something to the Ring. Domination and the breaking of other's wills was already natural to him, and these were the qualities of himself which he put into the Ring. And by the time he had made the Ring there was no truly good trait left in him for the Ring to take even if it could.
So this brings us back around to the original question which asks us to define what true evil is, precisely. In this essay I have focused mostly on the idea of evil as an absence of good, or that it is internal to people, but the Ring can fit into multiple modes of thinking. As Tom Shippey points out on page 142 of The Road to Middle-Earth, the Ring corrupts people both passively through the desire people feel for it and actively by its "betraying" of its bearers. There is a contradiction about, for the evil in its bearers seems to come about through their loss of whatever the Ring takes from them, but there can be no denying that the Ring must be something of an active force of evil, thus external, in order to bring about that change in it bearers.
Perhaps we can see evil as domination, as making things changing the nature of things (we have discussed previously about how evil and change are so closely associated, especially in the case of Melkor), particularly in the sense of removing a being's good qualities. What is domination if not the ability to shape a thing's very nature? And that is what the Ring is: a dominant force with the power to alter the nature of those who hold it.