A Great Opportunity Lost

I have been a fan on this site since December last and a fan of Alien since 1979. I watched the movie last night and was disappointed. However my disappointment was not related to the fact this is not an Alien prequel. Thanks to all the leaks and discussions on this site I new what was coming. But my disappointment was centered around a missed opportunity. I believe the basis and theory behind the movie were excellent but it was poorly executed to the point at times of lacking any credibility. There is so much wrong with this movie it just beggars believe Scott was howling on about it for the past year. I can only surmise that the Lindelof script and the pressure for mainstream-ness box office success lead to the bastardisation of the final cut. If this is the case it means we will never see the likes of ALien or anything remotely like it again. It also does not bode well for P2 already muted. Because the franchise which was suppose to be resurrected here is now in doubt. It may well be 10 years before we see the sequel. ANYWAY my questions are this. WHY DO THE ENGINEERS WANT TO KILL US (I know this has been mentioned before but there is no link to the steeling of the ampule by David). Shaw and Janek seem to come to the conclusion before they meet the Engineer that this is dooms day. Are we saying that after 35,000 years it just so happens we arrive on their planet and they are ready to move. THE OTHER QUESTIONS IS how does David know so much. It makes no sense his level of knowledge and also lack of protocol on the ship.