beat me to my theory on why Ben killed Locke.
I think Ben's original plan was to use Locke to get the Oceanic Six back to the island, as he knew they were destined to go back and as long as he was in on the circumstances, he hitch a ride--his main objective. The only problem for Ben probably could've manipulated everyone into going back the way we saw him to so, other than Sun, as she wanted to kill him. However, he could use Locke to get to her. But when he finds out that Jin is still alive and that Locke has the ring as proof, he realizes that he can use this information to get Sun back. He knew Locke needed to go back as well, but as long as he was alive, he was now connected to Widmore. Safer to bring him back as a corpse.