Jacob is something like God: he's trying to bring the best out of people by teaching them to do good and putting them through experiences. He taught young Kate not to steal. He exhorted Sun and Jin at their wedding to keep their love alive. He encouraged Hurley that he was not crazy but was blessed. He gave Jack another way to look at his father's advice after Jack had frozen up during a surgery.
In some places, this interpretation of Jacob does not work. Why did he encourage little James to write his hostile letter to Mr. Sawyer, who had contributed to the deaths of James' parents? Why not teach him to forgive? Was that to set the stage for the other guy to tell young James that "what's done is done," so he should put the past behind him and make a life for himself? That advice stayed with James later in his life, when he chose to stay on the island rather than going back to stop the deaths of his parents.
Why did Jacob cause the death of Sayid's wife? No, he didn't murder her, but he asked Sayid for directions, so Sayid was distracted from saving his wife from getting run over. Does Sayid especially need to learn lessons on the island, and he'd be less willing to go there if he had a happy life in the real world?
Jacob on LOST is more like a Jewish God than a Christian one, in one sense. Jacob's adversary believes that humans are morally corrupt and will only destroy one another once they arrive at the island. Jacob, however, maintains that people can become better. So I guess that Jacob's adversary believes in original sin (a Christian idea), whereas Jacob thinks that people can be encouraged to yield to their good impulses (a Jewish notion).
What is the motive of Jacob's adversary? Why does he want to kill Jacob? My hunch is that he's sick of Jacob bringing people to the island! Jacob is doing so to help people progress, but Jacob's adversary is tired of people coming to his home and messing things up--over and over again.
I think that Jacob's adversary is behind the smoke monster and the apparition of Ben's step-daughter, who told Ben to do whatever John Locke said. There is no more John Locke, since he is dead! The person Ben believed was John Locke turned out to be Jacob's adversary. As for the smoke monster, as I said, Jacob's adversary is sick of people coming to the island, so it makes sense that he'd set up some protection for his valuables (e.g., the temple). It's like Frasier not wanting people to touch his expensive objects when they waltz into his apartment.
Jacob was somewhat like the evangelical or charismatic God at the end of last night's episode. Ben asked Jacob why he had never gotten to meet him, and Ben was also upset because he had cancer, notwithstanding the island's healing properties. Ben had worked so hard for the island and had gotten nothing in return, and his question was, "What about me?" And Jacob's response to Ben was, "What about you?"
How dismissive! That's sometimes how I feel God is in evangelical settings. Some people experience God deeply in that they hear from him, receive answers to prayer, get assignments, etc. And others don't, even if they may try hard to believe and to do the right thing. Does God play favorites? Often I feel that he does. And there are plenty of Christians who are eager to put down a person who asks "What about me?" "The fact that you even ask that question shows you have a pride problem," they imply. That's what I heard in Jacob's response to Ben!
On the other hand, I can also picture the evangelical God being much nicer to Ben, affirming his love for him, even when Ben's path appeared dark. There are evangelical jerks who believe in a cold, stingy God, but there are also evangelicals who hold fast to God's unconditional love. In Jesus' parable of the Prodigal Son, the older brother asked his father, "What about me?" He had served his father most of his life and received little reward, so he envied his prodigal brother who had wasted his inheritance and received a warm reception when he returned home. The father's response to the older brother was not "What about you?" Rather, he affirmed his love for the older brother. And that's what I wish Jacob had done for Ben.