Yeah, it's me again, with another "hopefully" polemic discussion! :)
So what do we know? Engineer + black goo = steering the whole course of evolution of earth, into us. Right? the engineers DNA and evolving it into us.
So in LV-223 it's breaking worm DNA and making the Hammerpede right? ... WRONG!
Take a look at the substance david removes from the urn. Isn't it awfully similar to an Alien egg? Could it be that it's Broken down Alien DNA + Black goo. And if that is the case, Urn + worm does not equal better worm, it means the biological path to a facehugger.
Urn + worm = Worm like facehugger
Urn + sperm = Sperm like facehugger
Urn + Fiffield = Fiffield like Facehugger or xenomorph (take a look at his movement, all weird and confusing, like a crab trying to walk like a human)
Why is it building facehuggers? because it's the first biological step to make a full grown xenomorph.
Engineer + goo makes us
Alien DNA + Goo makes them (and it achieves it in the end of the movie)
Just my theory though, feel free to share your thoughst.