After reading through all these great theories and continuous pondering about the amazing lore and possibilities revolving around the film's universe, I've really had to change my thoughts behind the "connection" between Prometheus and Alien, and also the answer to Shaw's question about what humanities purpose was and why the Engineers changed their minds. Originally upon walking out of the theater I reflected on Prometheus as its own standalone film. There's enough within this film's own solitary stories to provide us endless speculation on the various interconnecting ideologies, philosophies, and inspirations. However, as a huge fan of Scott's prior masterpiece Alien, the very inkling of an idea that the universe is connected is just too tempting for to not try and find a connection between the films. Anyways, my initial thought was that perhaps the Engineers used the black mutagen (or as I call it "life serum") to create the original face-huggers/xenomorphs as a form of infestation based biological weaponry against whatever opposing force they may be facing (most likely in my opinion amongst each other). However, sacrificing one's own society to breed these monsters isn't exactly the most efficient or intelligent way of warfare. This is where humanity comes in. We learn in Prometheus that both humanity and the Engineers have the exact same genetic makeup. They created us in their image. What better way to build a hive of the most vicious killing machines on the planet than to simply produce a genetically identical twin species of its food source as a means of harvest? So basically, in this theory, the Engineers designed humanity simply as an efficient means of breeding the xenomorphs. This provides a shocking, horrifying, and perhaps faith crushing answer to the question of humanity's purpose. And this leaves several possibilities for why the Engineers decided that humanity ultimately had to be destroyed. Perhaps they did not intend for us to evolve as quickly as we did and we had rapidly become dangerously close to being a threat to them. Or perhaps the xenomorph had become increasingly difficult to control and there was a risk that they could somehow reach humanity and become impossible to manage. Honestly though, I believe that this theory is way too simplistic now and is obsolete. With all the seemingly endless possibilities involving the "life serum", the xenomorph appears to be simply one of the many organisms capable of being produced. Also, the end scene is fairly ambiguous. The creature appears to have a likeness to that of a xenomorph, but according to the various concept arts on the OP and the following explanations it may be something else entirely. Anyways, hope my rambling was at least a bit entertaining. I must see this film again.