Christian is definitively bad. There is no doubt about it in my mind considering every scenes he was in were creepy. Lost doesn't try to trick the viewer with symbolism, they are usually straigthforward and mean exactly what it looks like. That's why it's possible to make theory about the story.
I believe Jacob is "good" (opposite of Christian) for two reasons. The first one is that Lost always build its story around opposite, and you need something to "opposes" Christian. The second reason is that Jacob ditched Ben for Locke, and Locke is definitively a better person out of the two.
About the smoke monster, my guess right now is that it's a collective [un]consciousness of every dead people who came in contact with the island at some point. I don't know the details, but it probably has something to do with the resentment of people who wanted to change their destiny but failled...or something similar to this.
It would explain why the monster can take the shape of anyone who died at least.
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