After recent events concerning the Monster's interaction with Richard, Sawyer, and Sayid, I am starting to think that the whole Jacob vs. Monster situation is a classic portrayal of God vs. Satan. No big surprise there, but what interests me is the details and allusions that can be inferred.
The Monster offers something to everyone he encounters. He dangles these things (respect to Richard, answers/escape to Sawyer, and presumably Nadia to Sayid) in front of them-- for a price. Join me, and you'll have this glittering prize that you took for granted as unnattainable. "Join me" means a lot of things in the context of what's been going on. It also means "make a choice". It also means "abandon Jacob".
I have never subscribed to the idea that Jacob and/or the Monster represented any variations in the free will vs. destiny debate. I believe both of them offer choices of free will to the characters they encounter, and I do not believe that destiny and free will are mutually exclusive concepts. Destiny may be something as simple as a variation on Faraday's "whatever happened, happened" theory. We already know that Truman nuked Japan in 1945. He still made a choice to do so. We just have the luxury of looking at his choice from a different point in time, which is how I believe destiny and free will can coexist in LOST. Time travel as a concept has been introduced, and now we can question what "destiny" really means in the context of the show. Is destiny merely someone having the ability to look at our present from a future point in time?
Back to the free will side of things: Jacob offers choices explicitly on several occasions. The Monster implies it quite often, and even spells out THREE choices for Sawyer when he shows him the cliffside cave.
What is most fascinating to me is how closely recent events seem to parallel The Last Temptation of Christ. Jesus Christ believes he is destined to die in order to save humanity, and comes to accept it. Satan offers him a CHOICE for a way out of it:Get crucified and die, or abandon any notion of being a Savior, and live as a normal man. The device used to present this option to Jesus in the film is... a Flash Sideways Timeline! Jesus living as a normal guy with a wife and children. He is only shaken out of this when one of his former Apostles confronts him and points out that the timeline he is living in is NOT what was meant to be. This seems pretty similar to the FST we're currently seeing, and why Jack seems a little out of sorts and having subtle bleed-through memories of the OT.
If we can look at Jacob as a metaphor for a benevolent supreme being (God, to put it simply), and the Monster as some version of Satan, the issue is not necessarily good or evil, it is a question of temptation and choice, the Devil's oldest and best trick.