I have read many posts here r.e. the Engineers in PROMETHEUS, but what stands out is that the Robed Engineer in the beginning of the movie is SOMETHING DIFFERENT.
The opening scene Engineer is dressed in a Robe, while the other Engineers are dressed in military armor of some kind. This implies that there are two different factions within the Engineers.
The gist / theory that I got from the movie was:
The Engineers are divided into two factions - peacemakers and warmongers. The one in the Robe was a Peacemaker / Priest who sacrificed himself by drinking a heap of the black liquid (or an extra strong dose). It is implied that if the black liquid cannot cause a transformation successfully then it destroys the host (q.v. the exploding head), and the robed Engineer appears to have ingested enough to completely overwhelm his own system, disintegrating it but causing DNA in the local environment to rapidly mutate and spawn new DEADLY life forms. His apparent (again theorized) goal was to use the Black Liquid against the warmongers, to STOP them from their deadly mission to destroy Earth by sacrificing himself.
I support this idea with the implication that:
The Engineers who came to Earth were well received and apparently entirely beneficent. This suggests that the one's who seeded Earth may well have been a totally different political faction than the warmongers who seek to destroy Earth, e.g. the standard Hawk v Dove arrangement, or Priest-Scientists v Military, etc. The scene where the Engineers are running for their lives inside the ship implies that they are under attack. The timeline suggests that it would be from something scarier and / or more unpredictable than themselves, hence the Robed Engineer's monsters appear to have succeeded in destroying most of the warmongers and stopping them from heading to Earth.
QUESTION: Does this mean the Robed Engineer was on the side of Earth?