King Lear is an incredibly cruel play, and many of the characters are absolutely pitiless. Yet a few characters show extraordinary sympathy towards others' suffering. The human capacity to feel for others survives even the most desperate of moments. Yet what we see in Lear is that compassion is usually based on some sort of obligation – such as loyalty or family ties. Interestingly, these loyalties and these ties are the same causes of the extensive treachery displayed in King Lear.
The existence of compassion in King Lear prevents the play from presenting a completely negative view of human nature.
In King Lear, pity is a useless human emotion; it doesn't help any of the characters through their suffering.