The central plot device of The Lord of the Rings is the Dark Lord Sauron’s Ring of Power, which corrupts anyone who tries to use it, however good his or her intentions. The Ring is what Professor Tom Shippey calls “a psychic amplifier,” which takes the heart’s fondest desires and magnifies them to idolatrous proportions. Some good characters in the book want to liberate slaves, or preserve their people’s land, or visit wrongdoers with just punishment.
These are all good objectives. But the Ring makes them willing to do anything to achieve them, anything at all. It turns the good thing into an absolute that overturns every other allegiance or value. The wearer of the Ring becomes increasingly enslaved and addicted to it, for an idol is something we cannot live without. We must have it, and therefore it drives us to break rules we once honored, to harm others and even ourselves in order to get it.