As we know, in Season 5, the Oceanic 6 returned to the Island. I was quite disappointed at the vagueness of their motives for doing so, as John wanted to bring them back to 'save the Island', but it's never been clarified just how or why. John was told this by Richard Alpert during the Timeshift, and as of the The Incident, Parts 1 & 2, we discovered that Richard was in turn told by Jacob's mysterious nemesis. So, basically, it was Jacob's nemesis who brought the O6 back, to 'save the Island'. But what does he mean by that? Well, I think it's killing Jacob that he speaks of. At the beginning of the Season 5 finale, Jacob and his nemesis argue about whether people should be allowed on the Island, Jacob thinks yes, the Nemesis believes not. So, maybe the Nemesis has manipulated John Locke into leaving the Island, so he can bring back the Oceanic Six, so he dies in the attempt (at the hands of Benjamin), so his body and the O6 are on Ajira 316, so it crashes on the Island (because of the proxies Eloise was urging them to create i.e. Hurley's guitar case= Charlie's guitar case, and the other similarities between 815 and 316), so the Nemesis can take the late John Locke's form, so he can manipulate Benjamin into stabbing Jacob, Norman Bates style, in the foot of the Statue of Tawaret. So, the Nemesis has 'saved the Island' from Jacob, or so he believes. Personally, I think we haven't seen the last of Jacob. Because obviously, Jacob isn't a regular human being who can be killed by a knife. He's seen multiple times off the Island, guiding Jack, Kate, Hurley etc on their respective destinies. He's even shown bringing John Locke back to life after his spine-crushing fall from the skyscraper at the hands of the World's Best Dad Since Josef Fritzl. So, in a nutshell, he can leave the Island whenever he wishes, he doesn't age (like Richard) and he has the power to bring back the dead. So, surviving a knife wound and a bit of burning shouldn't be too hard for him. Maybe Season 6 will be the final battle between Jacob and the Nemesis, with the Oceanic 6 and indeed everyone on the Island used as pawns in the Battle for the Island.