I think Locke will not keep the promise. But not for the reasons above.

Remember that Locke was fooled by Ethan. He confesses to Jack that he feels in part responsible for Charlie and Claire's kidnapping because he worked "side-by-side with him (Ethan) and I hadn't felt even a twinge that he was not be one of us."

Here, they (the Others) went and did it again. Remember how mad Locke got when Henry tried those mind games a second time "How can you let him talk to you like that?" and the look he gave Henry when Henry did that little riff on what he would have done with the drawing of the map if he really was one of the others.

Locke looked like he was considering nailing him right there.

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charlie98210
"Whoa!" she said. "Cry 'Havoc!' And let loose the dogs of war!"
And, like Banquo's ghost, they just won't go away.