This theory regards the encounter between Ben an Widmore in “The Shape of Things to Come”. In the previous episode (“Meet Kevin Johnson”), Tom tells Michael that he cannot die, as the island won’t let him. During their encounter, Widmore asks Ben: “Are you here to kill me?”, to which Ben replies: “You and I both know I can’t do that…”
Thsi implies that, like Michael, Charles Widmore cannot be killed, as the island has not finished with him yet. This foreshadows a return to the island for him.