I think Ben is manipulating Locke (again) to gain more freedom and leverage. Locke doesn't let anyone comunicate to Ben (like Clair), so Ben is telling him about their common enemy to gain that trust. Like he said, he always has a plan and making moves to increase everyone's questioning of Locke is a good start. Locke saw a video, but only a brief scene and is only going off what Ben told him.

I do think Ben is evil, all of his actions demonstrate that. What good has he done that didn't somehow benefit him?

I'm sure that eventually Desmond will hear about what Locke saw and confirm Locke's suspicions, but it's possible that Widmore is trying to get to the island for a good reason .

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-- KB