The engineers at the beginning weren't on earth, it was misdirection. The researchers sssumed falsely that they were the creators only to discover that they were genetically and morphologically human, and so evolved on earth and were related to all other life on earth. Instead of finding god they found uplifted humans who stole fire from the gods (black goo etc) and were trying to be gods themselves when things went wrong. History then repeated when David the android stole fire from the gods in an attempt to himself become a god, to the same disastrous effect. A cycle of children playing with fire then getting burnt.
That leaves the question of who the gods are, and it seems the real space jockeys which I like to think of as some sort of Lovecraftian horror.
As for what the goo creates, I think it depends on what type of goo. Various creatures including the Deacon, Xeno, xeno eggs and facehuggers were depicted in the various murals while the vase-like containers had differing writing on them and some were smaller than others, so obviously there were multiple types of goo each of which would create different creatures.
The ampule David took was a Deacon creation one. The goo the Engineers used at beginning of the movie might have been designed to create other beings. Them infecting water supply could mean it was an attack on local beings even.
Oh, and the planet of the Engineers had females IIRC. Their strange appearance might be due to being adapted to space travel or even inclusion of tiny doses of goo to alter them. Perhaps the goo can be used to make humans adapt to new environments and that tiny amount of Xeno biomechanical material creates the features, which explains why human plus Xeno made hybrid baby that had engineer features
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