I know there are a ton of theories out there, but I just wanted to throw this one out there and see what you guys think. I think Jacob's enemy/rival could be the smoke monster, and here's why.
First off, the John Locke that was alive and that told Ben to kill Jacob seems to be Jacob's rival from the beginning of the last episode of season 5. This Locke says that he found a loophole, which is exactly what the rival said he was looking for. Also with Locke's body really being in that container the whole time, it's obvious that the Locke that's walking around is not actually John Locke.
Second, the smoke monster (looking like Alex) told Ben that he had to do everything that John Locke said, which is very convenient if Locke is the rival/smoke monster and needs Ben to kill Jacob.
Third, if I'm right, the rival/smoke monster (looking like Locke) is actually the one that told Richard to tell the real Locke that he was going to have to die in order to bring everyone back. So the real Locke thought that he needed to die, but in reality it was just the rival/smoke monster influencing him to kill himself in order to obtain his body so it could be used to kill Jacob.
We already know that the smoke monster can imitate people, and I don't know what else could cause there to be two John Lockes.
Implications: If the rival really is the smoke monster, then could Jacob be something similar? Perhaps a cloud of while smoke to keep with the show's theme of duality?
Problems: Did the fake Locke ever see the smoke monster in season 5? If they were ever on screen at the same time, that would disprove my theory. They were close when Ben was threatened by the smoke monster/Alex, but it's very possible the the rival/smoke monster just quickly moved around.
I hope this makes sense to someone haha I typed it and even I'm pretty confused
Hands, like secrets, are the hardest thing to keep from you
Lines and phrases, like knives, your words can cut me through
Dismantle me down (repair) you dismantle me, you dismantle me
