I believe Ben killed Locke because he was of no longer any use to him when he told him who he needed to talk to. I'm betting he's gonna get an unexpected awakening when he wakes up and locke is sitting over him. No doubt he'll make up some lie though.
When Mr. Widmore talked about a war coming... and locke needed to be on the island, I think this is the war with the island natives and the darma. I think he knows somehow that locke would be sent back to the 70's, before the purge happens.
I'm still not sure what side widmore is on though. He wants locke to be on the good side.... but what side is that? What side does he want to win? I'm guessing it's the side of the "others" since he worked with them for so long.
All this show we have been lead to believe that the others were the "bad" people of the island, and the darma was the good side.
The more this show goes, it seems like that role is finally being changed.
In the end, I think widmore will be shown to have been the good guy, and ben the evil.