This is just something that I thought about that might be a little funny if it were true.
What if Locke told Ben that he was going to kill Jacob just to see how he would react? It could be some kind of test to truly find out where Ben's allegiances lie. Ben spent so much time manipulating Locke and making him think certain ways, maybe Locke has decided to have a little fun with Ben. He hasn't mentioned killing Jacob to anyone else...and of all the people to divulge a plan to, Ben would be last on my list.
Just a thought, we'll find out soon enough I guess.