I think When Jacob told MIB that they were coming, he wasn't talking about the candidates the candidates, he was talking about Charles Widmore and his crew, maybe Charles wanted to kill both, Jacob and MIB, and take the Island for himself, but when he arrived, half of the job was done.
As for the candidates, he wants to protect them in order to choose one he can easily manipulate and do whatever he says, because he knows he is not a candidate.
What do you think??