In Treasure Island how does Stevenson forewarn the readers that Jim's fantasies will turn tragic?

Robert Louis Stevenson, in his novel "Treasure Island," uses the black spot on Billy Jones and his death as well as the revelation of the untrustworthy character of the men Jim came in contact with to forewarn the readers that Jim's fantasies will turn tragic. Only one person, Smollett, the captain, was trustworthy and Jim was little by little exposed to the evil acts that he witnessed in silence and in hiding. With all the negativity around him, he was sure to fail in the quest for the treasure.