IMHO, Alien/Xenomorph being created by engineers makes absolutely no sense
Humans are the Engineers' creation to elevate them to a godlike status, yet they only planted the seed for their beginnings. How and why would they go through all that effort to create the Xenos when they didn't/couldn't create "ideal" humans?
The black goo is already a perfect weapon for wiping out humans or any other species. Why would they feel the need to make Xenomorphs? Thus, it's more likely the black goo is a product resulting from experimentation with Xenos, not the other way around.
Alien Romulus shows that the black goo is relatively easily extracted from Facehuggers. It's far more likely the Engineers have done the same. Especially considering that humans evolved from the black goo, yet humans don't have any black goo inside them. If Xenomorphs were created from the black goo, I think it would involve some extremely difficult reverse engineering to extract the black goo from Xenos.
The Xenomorph life cycle always involve a host. If they planned to eradicate humans with the black goo, would they really go through the trouble of kidnapping humans and using them to develop the Xenos? And if the Engineers are a peaceful species, they certainly wouldn't experiment on their own kind to produce the Xenos.
If the Engineers wants to wipe out humans due to their violent nature, why the hell would they make the most violent specie imaginable??
I think it's far more plausible that Xenos are a parasitic species that plagued the engineers eons before they used the black goo to spread life on Earth etc. A perfect organism produced by harsh conditions and eons of evolution.
Xenomorphs were probably seen as a force of pure evil by the ancient Engineers, but as their civilization progresed, got cocky and believed they could control the power of the Xenomorphs. I think it fits the whole theme of Prometheus well, showing that even our creators were merely playing God with a power they couldn't fully control.
I think it'd be cool if David recreated the Xenomorphs after the Engineers driving them extict, but seems like it's really hard to fit in with the Engineer ship on LV-426. I think it's most likely the Space Jockey on LV-426 were transporting Xeno eggs for research/production purposes to create the black goo.
Feel free to disagree, I'm happy to hear counterarguments. Cheers!