As Collider put it, Prometheus revealed that "[the Engineers] were headed [to Earth] to wipe out humanity when the organism they created turned on them, leaving behind the deadly tomb that the Prometheus crew finds." Dr. Elizabeth Shaw named them the Engineers in Prometheus, as she was under the impression that they had engineered life. The movie ends with David and Dr. Shaw lighting out for the territory of the Engineers' homeworld, the goal being "to better understand why they wanted to snuff out their creation of humanity," or so it seemed.
In May 2017, Ridley Scott corrected this common misconception, saying, "So we've reinvented the idea of Alien, I think, which is that Covenant gets us a step closer to who and why was this thing designed to make human beings. And if you think it's them [the Engineers], you're dead wrong."
Well, great. So, everything you've ever known about these big blue guys is a lie. Next thing you know, the black goo will turn out to be a mutant creatine supplement, weaponized to put a stop to doping in the Mr. Universe contest.