This may also have to do with what Isaac told Rose in "S.O.S.": that different places have different sorts of energies, and his home did not have the right sort of energy for her. It could be that the Island does not have the right kind of energy for Ben; it's sort of energy could even have a malignant effect on him, potentially having caused the tumor.
Additionally, the Island may have been keeping Ben alive, but the discharge has taken away the electromagnetic energy that let him live.
Ben infiltrated the Losties camp because he was going to die anyway, despite being the Others's leader.