Regarding why Ben killed Locke, I have a number of conflicting ideas. First of all it's quite likely that he knew Locke would actually try to fulfil his promise to Jin not to bring Sun back if he remained alive. Secondly, after watching the scene twice, it looks like he has no idea how to get back to the island until Locke tells him. One possibility is that Hawking is actually being employed by Widmore to help the O6 get back to the island, with no idea that Ben is supposed to be kept away from it. However, the real change in his mood seems to come when he learns that Jin is still alive. It is possible that these two tidbits of information combine to cause him to decide to kill John, although I am still uncertain as to why this would be.

I don't think he expected Locke to be resurrected, however.

As for Widmore's claim to have protected the island "in a non-violent way", that's obviously bull given his order to chop off Juliet's hands. I wouldn't trust anything either Ben or Widmore says; both of them are pretty clearly more interested in their own power than in telling the truth.

The labyrinthine twists and turns of this show really are coming to match those of the Illuminatus! trilogy, an acknowledged influence on the series. It's also where the number 23 comes from, although the creators haven't explicitly acknowledged that part yet.