Is it me....is it that Ben was not as knowlegeable as he claimed. We know that he had never communicated with Jacob although he pretended to. Ben did not fully understand the rules if he did he would have known that Alex was in real danger. Ben didn't know about the candidates, he wasn't a true leader, Jacob didn't appoint him he took the power, therefore he wasn't brought in the "need to know loop" He is very cunning and used his smarts to gather information, thus learning that certain people are hard to kill.
He thought that because she was born on the Island that Alex was one of the protected and totally believed she wouldn't die. What Ben didn't know it was Danielle who was the candidate not Alex.