Ben was a candidate by Johnzo

Let's think about candidates and candidate succession. We have a our present day situation with Jacob needing a replacement and our six, scratch that, three candidates remaining.

In the Across the Sea, we had Mother (Allison Janney) who needed to be replaced and the two candidates were Jacob and the man we know as MIB. She was killed by a candidate, the MIB. She thanked him for killing her. She was happy she could finally leave as if she herself were trapped on the island. In fact. she very much expected it and did not fight it, exactly like Jacob did.

Back to present day. Ben killed Jacob and I think the only reason Ben was able to kill Jacob was because Ben was or is a candidate. Candidates are people Mother or Jacob brought to the island. And I think only candidates can kill Jacob.

Although I do not think this theory really changes anything in the way of story, I think its the just the rules of the game.