Just caught the movie and was reasonably entertained.
Is there any info that came out before or after the film's release that explains:
1.) Why the man in the beginning killed himself?
2.) Why original mankind wanted to kill Earth mankind?
3.) Why David put that biological shit in the one guy's drink?
Regarding 1 & 2, I was kinda just assuming from the whole DNA destruction and creation in the very beginning that he was the source of Earth humanity by way of some strange biological agent he created. I was thinking maybe the other original humans saw that guy's Earth mutant human creations as an infection or disease in need of curing before we cause something bad (like Earthlings infecting the universe or some shit like the beings feared in the Alien films). But it's all just kinda conjecture. That theory didn't explain where those cave paintings came from, though. You'd think the cave paintings were from repeated visits, but if the original mankind was upset at the creation of new humans, they wouldn't have come back to hang out and be known. idk.
Regarding 3, I just assumed he did it because he didn't like him...but he never explained it. Not sure if he expected or intended that the girlfriend would get pregnant. I guess I'm just not clear what his motivation was, given he doesn't have emotions that one would traditionally link to angry, spiteful actions.
Everything else I could resolve. The only part that really irritated me was the first two dudes that got killed with the one guy sweet-talking an alien clearly in "Cobra, ready to bite you" mode. Otherwise, it was good enough.
This post represents a combination of things said in interviews with Scott and the writers as well as speculation that has been pretty well established in internet-nerd Prometheus discussions. I can elaborate further on any of these points if you want me to.
1) The opening scene depicts one of the Engineers sacrificing himself in order to seed life on an Earth-like planet. Scott and the-writers have said specifically that it was meant to be ambiguous as to whether this was earth or not in order to imply that the Engineers have done this repeatedly across the universe. Further, this scene is meant to show that the engineers' society/culture/religion values sacrifice a high virtue. They sacrifice their own lives in order to seed life on other worlds.
This is meant to directly contrast with Weyland. Weyland has reach an unnatural old age, to the point that he resembles an animated corpse. He leveraged the combined knowledge of mankind in order to mount his mission on the Prometheus simply in hopes of prolonging his life a little longer. These two contrasting worldviews are brought to a head in the scene in which the engineer kills Weyland. When confronted with this selfish old man the Engineer is disgusted and end his life. He kills of one of his people's creations, fittingly using Weyland's own creation as the means.
Another important note on the opening scene is that this is meant to depict the seeding of life in general (a sort of terraforming) and not specifically the seeding of man. It is separately shown in the first act of the film that the engineers had returned to earth many times (the cave paintings) to guide our path over the coarse of our early development and probably guided man's genetic evolution as well.
2) This is the central mystery that sets up the sequel. Shaw, a woman of faith, is left wondering why those who created mankind at somepoint decided to exterminate us after having previously guided our path. In interviews Scott has said that thematically this turn was meant to represent original sin or potentially even that this is revenge for the crusifiction of Christ. In terms of the actually plot and canon of the universe, whatever happened 'roughly 2,000 years ago' (hurr hurr) is a mystery and Scott has said that they very deliberately wanted to stay at arms length from directly tying this into human history/religion.
All you really need to know as a viewer is that the events of 2,000 years ago are still a mystery and this will be a large part of the plot in a sequel if it every materializes.
3) David is instructed to do this by Weyland, thus the scene is which Vickers yells at David to tell her what he told him. He only offers Vickers the cryptic reply of "try harder" but this is meant to be the direct catalyst for David 'infecting' Holloway. As viewers we don't know how explicit Weyland's instructions were but, regardless of what he said, this lead to David doing what he did. This ties back into what I said about Weyland in my first part of this post- his quest to extend his own life came with no regard for the lives of others.
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I hope some/most of that made sense. I am obviously pretty far down the rabbit hole of Prometheus so I have no idea to what extent this will be digestible by anyone.