So Hoax-Henry said he would help Locke, if Locke promised to keep him safe from the other losties. Locke opens the door and asks him who he is. He lies and tells him once again, he's Henry. Only then Locke makes the promises to him.
Hoax-Henry then goes on to help Locke and "save" him and the rest of the island by entering the numbers. (As far as Locke is concerned) Then stays and helps Locke when he could have run off.
Do you think Locke will now, hold to his promise and try to help Hoax-Henry? Or, do you think Locke will say, Forget it! You lied to me, so the promise is void. Dispite the fact that H.H. held to his side of the bargain?

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Hope is a very dangerous thing to lose. - Sayid

I do what my Rice Krispies tell me to. - Anonymous