Why does the beautiful lady leave the knight at the cave?

The narrator happens upon a knight in a field, all alone and weak; he is, apparently, dying as "on [his] cheeks a fading rose / Fast withereth." The knight explains that he met a beautiful lady, who seemed like a "faery's child" with her long hair and wild eyes. He made her flower garlands and they exchanged vows of love with one another. The beautiful woman took the knight back to her home, an "Elfin grot," they kissed for awhile (and who knows what else?), and then she lulled him to sleep. While he slept, the knight dreamed of other men who have been seduced by this beautiful woman and who are now all dead. They cry out to him in his dream that she has him in her power too. When he woke up, he found himself all alone on the cold hillside, the mysterious woman nowhere to be found. We can assume that the knight will now die as well. It seems, then, that this woman is something akin to a witch, that she takes pleasure in seducing men and ruining them. There seems to be something of the supernatural about her.

The title of the poem translates to "The Beautiful Woman Without Mercy." The woman in the poem is a sorceress-like figure who attracts men to her and leads them to ruin. Her motive is never identified in the poem and she remains mysterious to the poems speaker and to the audience, however, it would appear that her actions actually lack motive. After the speakers ordeal, he tells shares with passersby his experiences with the bewitching woman and they all have the same reply:

I saw pale kings and princes too,
Pale warriors, death-pale were they all;
They cried—‘La Belle Dame sans Merci
Thee hath in thrall!’

This suggests she has left countless other men in similar predicaments for undetermined reasons. As the title suggests, she is simply a beautiful woman who uses her powers to toy with men and led them into ruin just because she can. Some critics see Keats poem as being inspired by his own unlucky history in love and others see it merely as a successful experiment within the ballad genre.