Like so much with King Lear, this is far from an easy answer. Much of it is going to depend on your own personal view of what is presented. The idea of evil destroying itself is representative of a Modern reading of the text. Evil is shown to be intrinsic to human nature. When one strips away the trappings of what human beings wish to project, they, like Lear, can be reduced to a "mortal worm." In the ending of the drama, the worms end up feasting upon and destroying one another. Evil becomes cannibalistic, devouring one another until there is nothing left, an idea revealed through a Modern reading of the drama.
If one views King Lear from a Classical or traditional interpretation, the perception of good and evil might be different. Certainly, the follies and frailties of human beings are intrinsic to consciousness. Yet, there can be something redemptive in Lear's narrative. In order for the natural order of good to present itself again, there has to be a complete eradication of evil in order to be restoration. In the name of Classical moral and ethical restoration, evil is destroyed. Whether it feasts on itself is secondary to the fact that there is restoration. This mode of thought suggests that human beings have learned from Lear's mistake and the absolute disaster present at the end of the drama enables a new moral and spiritual order of righteousness to emerge.