One question though that really isn't explained in the film. The opening sequence leads us to believe that the engineers came here and left permanently. However, the cave drawings and the similar ancient architecture leads us to believe they continued to visit for sometime as humanity 'grows up.'
So, why would they want to destroy their own creation as they watched over us presumably over thousands of years?
In myth, Prometheus makes man from clay, sides with the gods against the titans, and steals fire from the gods and gives it to man, only to be condemned for it. So, I suppose the "Prometheus" we see at the beginning of the film is part of one faction of engineers that creates and guides human life, against the will or law of their race, while the ones we see at the end of the film are part of another faction seeking to destroy their works. Their enemies. If the "good engineers" gave us fire, that is technology 2000 years ago, perhaps the "evil engineers" are out to destroy the perceived threat of us. Or maybe to them, all or them, Earth was just a petri dish that served its purpose, and now goes to decon.
Just a guess.
Oh, and A-. It was great.