I think David did what he always did in the film - acted within parameters for his own whim and curiosity. He is ordered by Weyland to "try harder" with their findings to produce results, and promptly, happily infects Holloway, who is an asshole to him, emblematic of how David seems to feel for most humans. This fulfills his programming objectives. So does what he says to the Engineer, word for word - but it's how he says it, and when he says it, that he suspects may have an adverse effect not in the humans' favor.
Or maybe he just meant to do as Weyland asked, and was happily surprised. But judging by his prior conversation with Shaw, I doubt it. The ambiguity of David in particular is one of the best things about the film and the franchise's future.
