There have been many interpretations of this poem. Mine is that the beautiful and cruel woman of the ballad is a personification of nature. It is easy to fall in love with nature, but in the end she is cruel because she claims you in death, as she does all the "death pale" kings, princes and warriors.
I saw pale kings and princes too,
Pale warriors, death pale were they all,
They cried -- "Le Belle Dame sans Merci
Hath thee in thrall!"
We are all destined to become part of nature again--to be slowly absorbed into the soil, the roots, the trunks, branches, leaves and blossoms of the trees we loved while we were alive. John Keats died at an extremely early age. He was only twenty-five. Much of his poetry is haunted by his feelings about death, and "La Belle Dame sans Merci" is a prime example. Those feelings are not of fear but of a characteristic mood. He always sees nature through a veil of melancholy. He has seen his brother die of tuberculosis and knows what to expect for himself. He is tormented by the thought that he has to leave the world in which he finds so much that is beautiful for him to love, not excluding Fanny Brawne, the beautiful girl he loved.