I got the feeling that Ben was helping Locke stay alive to gain information. Locke said two things that made Ben pause. One was when he mentioned the name of Mrs. Hawkings and the other happened just before it (I think ... I cant remember what the information was though). It seemed to me that Ben didn't decide to kill Locke until John inadvertantly passed info to Ben (that he seemingly didn't know yet).
Thus ..... no more need to keep Locke around, since he has the info he needed.
I don't know. Sure is good television though.
