"The Island keeps the balance of good and evil in the world. It always has and always will perform that role. And the Island will always need a "Protector". Jacob wasn't the first, Hurley won't be the last. However, Jacob had to deal with a malevolent force (MIB) that his mother, nor Hurley had to deal with. He created the devil and had to find a way to kill him even though the rules prevented him from actually doing so.
Thus began Jacob's plan to bring candidates to the Island to do the one thing he couldn't do. Kill the MIB. He had a huge list of candidates that spanned generations. Yet everytime he brought people there, the MIB corrupted them and caused them to kill one another. That was until Richard came along and helped Jacob understand that if he didn't take a more active role, then his plan would never work.
Enter Dharma which I'm not sure why John is having such a hard time grasping. Dharma, like the countless scores of people that were brought to the island before, were brought there by Jacob as part of his plan to kill the MIB. However, the MIB was aware of this plan and interferred by "corrupting" Ben. Making Ben believe he was doing the work of Jacob when in reality he was doing the work of the MIB. This carried over into all of Ben's "off-island" activities. He was the leader. He spoke for Jacob as far as they were concerned. So the "Others" killed Dharma and later were actively trying to kill Jack, Kate, Sawyer, Hurley and all the candidates because that's what the MIB wanted. And what he couldn't do for himself."