Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Bleak
'Nuff said.
King Lear is a dark play, and its tone reflects this. The powerful language of Lear's cursing of his daughters defines the play, and as Lear goes mad, he begins to curse the entire social world and the entire universe, even:
Rumble thy bellyful! Spit, fire! spout, rain!
Nor rain, wind, thunder, fire are my daughters.
I tax not you, you elements, with unkindness.
I never gave you kingdom, called you children;
You owe me no subscription. Then let fall
Your horrible pleasure. Here I stand, your slave,
A poor, infirm, weak, and despised old man.
But yet I call you servile ministers,
That have with two pernicious daughters join
Your high-engendered battles 'gainst a head
So old and white as this. O, ho, 'tis foul! (3.2.16-26)
This language of rage mixes with the language of madness, reflected in both Lear's speeches and in Edgar's fake mad ramblings as Poor Tom the insane beggar. Lear's crazy rants against the human abuse of justice and power are so brilliant they lift the tone of his later scenes to pure philosophy.
The language of nihilism also reverberates through the play, from Cordelia's first refusal to say anything but "nothing," to Lear's final cry of grief that his daughter will never breathe again. "Never, never, never, never, never," Lear cries out, in what some critics call the bleakest line of iambic pentameter ever written.