Well if the Engineers were created by god maybe they have the same problem that David has, no soul. Who does horrible things, who creates monsters? Soulless individuals, that's who.
Weyland claimed that David couldn't appreciate all the gifts bestowed him because he lacks the one thing he'll never have, a soul. So God created beings without souls who then went on and created man with the intention of man having a soul. After so many failed attempts a race of beings were finally created that have a soul. This, in Gods discretion, made man perfect. The engineers of course hated that idea. That of God wants our women.
Yea the planet at the beginning of Prometheus can be planet anywhere in space. The scene was just there to show how they go around the universe spreading the seeds of life.