To label the play as a battle between good and evil would be to do Shakespeare a great disservice. No character in any of his plays are either good or evil. They are human beings. Shakespeare was a great observer of human behavior.
Lear is a very old man who has ruled for most of his life. He is used to everybody obeying him. He has three daughters. Cordelia is a late life child and favorite. She and her older sisters, Goneril and Regan, probably don't share the same mother. They have been married and living with their husbands for many years, it would appear.
Lear proves he is a foolish old man by asking a question no parent should ask a child: how much do you love me? Goneril and Regan play Lear's game. They flatter him and tell him what he wants to hear. Cordelia, his youngest, "...cannot heave her heart into her mouth." (What a wonderful image.) This is not what he wanted to hear. Her answer is unacceptable and she is cut off with nothing. For telling him the truth, Kent is banished. Only the Fool can tell Lear the truth, but he is a fool, so why listen to him?
Is it surprising that Goneril and Regan try to protect themselves against the whims of the aging and volatile king? If he can so easily disinherit his favorite child and banish a loyal retainer, what would he do if they displeased him?
The old king sets this all in motion, but is he evil? I think not. Are the sisters evil? Again, I think not. Do they do evil things and make terrible choices? Yes
Is Edmund evil? Again, no but he does makes some awful choices. When he calls on Nature, is he calling on a positive or negative nature? Flowers and trees or flood and earthquakes?
It is only in the storm scene where Lear strips himself to the elements and has concern for another human being (Mad Tom) that his redemption begins. He must travel through madness to discover what love of family really is before he dies.
The blinding of Gloucester (Act III, scene 7) is an evil action but the scene that follows (Act IV, scene 1) is touching and compassionate. An amazing contrast.
"The weight of this sad time we must obey, / Speak what we feel, not what we ought to say. / The oldest hath borne most. We that are young / Shall never see so much, nor live so long."
I would say that King Lear is more of a study of the collision between old and new orders of recognition. There is a good vs. evil element present, but I sense that the study of King Lear, as a character, is the examination of the collision between tradition and modernity. It is here where I think that the drama excels. I am not entirely certain that Shakespeare is making one out to be better or greater than another, but rather positing the inevitable tragic condition that exists in human consciousness. King Lear and the relationship between how he feels he should be received and how he actually is received is something that seems to culminate in the notion of self awareness and greater understanding of identity. It is here where I think that the play excels. Indeed, there is a good vs. bad configuration with Cordelia vs. Regan and Goneril amongst others, but I think it is in the collision between two differing and incompatible visions of what it means to be human where the drama impacts me the most.