The black goo

I have to say, I did like the new movie. I thought it was pretty enjoyable, appropriately  gross, and even a little thought-provoking (like Prometheus had hoped to be).

One of the things that I didn't like,, even though the scene itself was cool, was the continued confusion over that goddamn  black  goo. In this movie, when dropped on the Engineers, it simply kills them.  Well, not simply. It looks like a pretty  painful death. My point is, he basically eats away at their bodies similarly to how the engineer at the beginning of Prometheus dies; although it was never clarified whether that substance is the black goo. For the sake of the conversation, I'm going to say that they are the same substance.

Things start to get a little murky when the crew of the Prometheus enters the Engineer's ship. In the room that contains all of the containers with the black goo, there is a mural with what appears to be a xenomorph. It is a logical assumption  then that the black goo creates xenomorphs. I think we're supposed to assume that is what Fiefield and Holloway were turning into.

This was confusing, because the mural on the wall is specifically a picture of a xenomorph. the other mural is an Engineer with it's hand on another creature. The logical assumption is the Engineers are the creators of the xenomorphs or at least  a very close cousin, since David did the tinkering.

Then again in the new movie, we see two different results from the same substance. One being the death of the Engineers, the other being the neomorph; which if I understood correctly was the result of the residue  that fell to the earth  mutating some local lifeforms.

Why does it destroy one life form and turned the others into incubators for xenomorphs? I think Ridley attempted to explain it when he had David say  that it is unstable, but that's simply a bad excuse and really doesn't solve anything. It still does whatever the plot needs it to do..