Its possible the information David needed was in the engineers temple, and he just added that missing link, the non ethical missing link that is, i think the engineers create things that serve a purpose, us humans are cruel, violent and always wanna be the dominant ones at a high cost, so maybe the engineers created us and we killed one of their messengers 2000 years ago or whatever happened that pissed them off, or we are the creation of another race or another bigger alien type, just like the xenomorphs are also from that same creator,
we are related to the xenomorph, we are the inferior xenomorph.
Anyway i think David just created something in between, maybe the engineers are capable of creating something much stronger then the xenomorph.