I think that the smoke monster is the devil.
reasons - in John Milton's book Paradise Lost the devil is a fallen angel.
In the Bible another name for Jacob is Israel in hebrew this roughly translates as one who struggled with God. sometimes the origin of this name comes from biblical Jacob's fight with an angel - in Lost this translates as Smokey who is MIB and the New John Locke.
Also when in the Bible Jesus goes to the desert to fast he draws a circle in the sand around him to keep the devil out, the others use ash to circle themselves around to keep the devil out.
In the film Dogma Ben Afleck and Matt Damon are fallen angels trying to find their way back home - to Heaven.
When evil Locke says he wants to go back home he means he wants to got back to Heaven.
So the Island must be some sort of prison for Devil? but I think Jacob is their to guard him - I don'y know why Jacob brings people to the Island - but the poem Paradise Lost - is all about destiny and free will the reason why Satan fights against God is beacuse he dislikes the fact that he was created and is not self sufficient. Also in the poem - God emphasises that free will will be the end or fall of man. destiny and free will are key themes in the show and more so in the final season.
like the angels in Dogma smokey/satan is pessimistic about mankind but Jacob remains optimistic.
Also smokey is able to 'possess' dead people making me believe that he is Satan.
Why is Jacob chosen by the Lost people? well Jacob's Ladder is a way to get to heaven I think this is found somewhere in the temple? even though it is an egyptian temple. With Jacob dead I think Smokey/Satan can now gain access.
In the Bible Abbadon is the destroying angel who comes from the Abyss to destroy the world and is often connected with the Devil.
Remember Mathew Abbadon? he is working for Widmore - who recruits Miles but Bram the guy who is with Ilana on the Island on the Ajira flight who forsefully takes Miles in van in a season 5 flashback(Hoth I think) Bram says to Miles he is on the wrong side. So I think Abbadon & Widmore are in league with Smokey - they tell Locke that he must go to the Island in the first place and help him get back in a manner of speaking - Wimore told Locke that he must be on the Island as there is a war coming. in going back dead Locke ends the final part of the loophole that Smokey needs to kill Jacob