[Blloydy]

I think it's a combination of things:

1. He knows Ben knows things that would help him and the other Losties survive
2. I don't think he ever really wanted to hurt anyone, Ben seems to beleive he will die if he doesn't go with Locke and Locke might not want to be the (indirect) cause of his death. Especially if he will be better off with Ben and all of the information he offers.

I don't think he plans to get 'revenge' on Ben, afterall, he couldn't kill his father after what he did ti him.