I agree that Prometheus didn't answer its questions in a fulfilling way, and that it never really intended to is all the more frustrating, but I disagree that many of the pertinent questions don't make sense. "Why did they create humans only to decide to destroy them?" Assuming the engineers on lv-223 actually did purposely create human beings, and let's assume they did, they either changed their mind about the worth of humankind and thus decided to wipe the slate clean with a new biological creation (guess what), or they simply wanted to create something else in an old petri dish just because they could, but it's important to remember that the question rests solely on what Shaw has extrapolated. It's the same as asking why some parents hate or don't care about their children, and David represents the inverse of this theme; where he wants his parents dead, and so while he does serve Weyland, it's clear what his ultimate motives are. Heck, he's the first to assure Holloway that the air is "perfectly breathable", thus his efforts to expose the crew to infection was evident before he dipped the goo in Holloway's drink. That this could put Weyland at risk wouldn't phase David; Weyland merely told him to try harder, and with that he could doubly serve Weyland and his own motives. Also, the star maps don't point specifically to lv-223; there would be multiple worlds they could have landed on in the star system, they simply chose an earth-like planetoid in that system, but alas it turned out to be anthrax island and not the home world of the engineers. That the star maps were an "invitation" is also just Shaw's assumption. I prefer Fifield's assumption - "bullshit".