Ben was for many years spoofing Dharma and pretending that their stuff was still on the island. They found out, got mad, and are looking for him. Eventually he's forced off the island by the boat people, forced to work for Dharma, and knows about the "six" who leave.
Why six? Because in the end so many castaways decide to stay. That, and well... things get bloody between Locke and Jack. Lots of people die.
But for those who stay, the island has become their home... Rose/Bernard had an episode like this explaining why they would stay. I also think the next person we'll see is Michael as a "sixer." He's the one in the casket.
I believe Claire gets in the helicopter (as Desmond predicted), but then something happens to her. An accident? Kate's involved? So out of responsibility Kate takes on Aaron.
Ben is holding this over Sayid's head. This is why Sayid is working for him - to stop Ben from blabbing. So Ben is using him to take out the Dharma people so Ben can go back to the island, take out Locke (who is still there), and regain control.
This means taking out the tall African-American gentleman who hired the boat people.
Then again, maybe it's just a TV show. ;)