The black smoke seems to be the rival of Jacob and the one who ultimately kills him. We know the smoke can take the form of dead people on the island, so the loophole that he found was to use Christian to get Locke to do exactly as he wanted so he would return dead to the island. Then, while his Locke form pretended to go away, he convinced Ben to do as Locke said and kill Jacob. I think the rival couldn't outright kill Jacob. He needed someone mortal to do it for him which is why he used Ben.