Why was Jim haunted by nightmares in Treasure Island?

When the old sea captain first came to stay at the Admiral Benbow Inn, he was quite silent and kept to himself. Jim noticed he would scan the area regularly with his spyglass, and eventually the captain asked Jim to keep a lookout for "a seafaring man with one leg." The old man paid Jim "a silver fourpenny" at the beginning of each month to keep watch for such a stranger. Because Jim took that duty seriously, and probably because it was such an odd request and the captain seemed to fear the arrival of the one-legged man, Jim started having bad dreams about seeing him.
During stormy nights, he conjured up various terrifying images of the disabled sailor. Sometimes he dreamed of him having a leg to his knee, and sometimes the leg had been amputated at the hip. Occasionally he dreamed that the man was an inhuman creature who was born with only one leg "in the middle of his body." In his nightmares, the mystery man would chase him over the landscape. Not only that, but the captain's favorite song about "fifteen men on a dead man's chest" became interwoven with his nightmares of the one-legged man, making both the song and the watched-for stranger even eerier. Because of his nightmares, Jim felt he more than earned his fourpenny piece each month.