I'm wondering what or who is preventing Ben from killing Widmore.
Never realized this thread was here ... Ok now I join the fun!
A friend of mine had a theory last night as we were discussing this week's episode, and I kinda agree with him.
Perhaps Widmore had been to the island at some point in the past (maybe the Black Rock?), and like Michael has been infected by the powers of the island, and can not die? This is why Ben can not kill Widmore, and has to use his daughter to get back at him.
There seems to be others who can not die or seem very very old (do not age) like Dr. Richard Alpert. The island obviously has some kind of healing/aging remedy for some people.
Also, I am not sure if Ben is "controlling" the smoke monster, but maybe just summoning it or aggravating it to use for his own benefit. He may have the ability to capture it using the CV (Cerebus Vents) we saw on the map in the hatch at some point.
My theory on the smoke monster is that it has some kind of "god-like" power and can read people's thoughts, and kills people who have a shady past, but not until that person's "job" is done on the island. Everyone has a purpose, just like some people believe that God puts them on Earth for a purpose and when that purpose is done, they die. Well the same with the smoke monster ..he determines when that person's purpose is done on the island and then kills them, if they have been "bad" in the past. Which is why it took awhile for Eko to get killed by the smoke monster. And it doesn't quite answer why he killed the Pilot in Episode 1, but I have a feeling the pilot has skeletons in his closet that just have not been revealed yet. His "job" obviously was finished - he got everyone to the island. It was his fate. Which is why the smoke monster has not killed Kate or Sawyer because they obviously have shady pasts, but their purpose is not fulfilled yet. The smoke monster has a reason to keep them alive. What the smoke monster is scientifically (material-wise), who knows ...hopefully the show's writers will reveal that by the end of the series, if they even know!