My perspective on Shakespeare'sKing Learis that the main theme is the painful fact that each succeeding generation pushes out the generation that came before it and was responsible for its creation. In order to evaluate the theme or meaning of the play, one would have to consider both the main plot involving Lear and his daughters and the subplot involving Gloucester and his two sons. It is natural for a father to love his children, but it is not necessarily natural for children to love their father. Each new generation has to fight to find and maintain a niche on the small, rotating globe and then reproduce, and this means displacing those who are trying to hold on to their little niches.
In John Keats' "Ode to a Nightingale" he uses the phrase "No hungry generations tread thee down." That is the truth about life: the hungry generations of all the various species, including homo sapiens, are forced to "tread down" those that came before them. When Lear and Gloucester get together in the open fields, after both have been displaced from their homes and all their property, they commiserate about the cruelty of life. It is a dog-eat-dog struggle for existence and reproduction. Shakespeare had a hard life. He knew what it was like to struggle for existence.
There are some good characters in King Lear. Edgar and Cordelia are the notable examples. Shakespeare may be using them mainly as "foils" to offset the wicked Goneril, Regan, and Edmund. There would have to be some good people in the world; otherwise we wouldn't recognize the wickedness, greed, and selfishness of the others. But the thrust of life is always for one generation to displace the generation that created it.
It is interesting that so many of Shakespeare's plays have themes related to the idea of usurpation. These include Hamlet, Macbeth, Richard II, Richard III, Henry IV, The Tempest,and AsYou Like It. In Julius Caesar, Brutus and Cassius seize power from Caesar and then Antony and Octavius seize power from them. In Othello, Iago usurps Cassio's position and may be hoping to displace Othello. King Learis about usurpation, in a sense, because Lear's daughters get his kingdom under false pretenses, and Edmund steals Gloucester's title and lands from both his father and his brother.
I agree with each assessment above and I'd add one more possible argument for a central theme. King Lear can be read as a play about identity.
Lear is doomed when he attempts to change who he is. It is not the crimes and ambitions of his past that lead him to failure and destruction as much as it is his attempt to do the impossible, to be what he is not.
Another interesting perspective came from a lawyer who once told me the King Lear is a good example of what not to do during retirement. He said that King Lear was crazy to give out his children's inheritance before he died because that is exactly what he has seen children do to parents in lawsuits. Children who are anxious to get "what they deserve" from their parents merely use them in the end. I thought that was certainly an interesting perspective of the parent/child theme.
I can't argue with betrayal, but I also think that a major theme is the ultimate humanity of us all. Lear is full of himself, claiming absolute paternal control over his daughters, which is why he gets so angry at Cordelia. This absolute control erodes very quickly along with any vestige of Lear's majesty as the consequences of his decision to alienate his daughter and the fact that he is surrounded by wicked schemers literally causes the world he has constructed for himself to come crashing down around him. It is not going out on a limb to say this, but I have come to regard King Lear as Shakespeare's finest play.
I think that the main theme is betrayal. The play shows how people (like Lear) can put their trust in the wrong people. It then goes on to show what the impact of betrayal is on the person who has been betrayed.