Words mean more than what they imply on the surface.

Ben did not gas D.I.
Richard Alpert's and his crew did it.
Since the relating episode was a Ben-centric one, many of us read the narrative as if it was solely for Ben and by Ben, with Ben's decision and initiative. Most likely,it was a larger plan (of Jacob, I don't know) and the timing was right for Ben to "reborn". So Richard made him a part of it which must have been to his (R's) advantage at some practical point too for the execution of the plan..

Ben manipulates more than he lies.
And which character on/off the island never lied really?
Of course, except for Vincent, the canine Chuck Norris..

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"Bear...? Is that you?"-- Hurley S03E4