Originally Posted by JD10367
So now that I've seen it, I'm a bit confused about the whole "the Engineers were planning to eradicate earthlings". There had been some references to the Engineer they woke up being angry, with some contention that it had to do with David and humans creating life as the Engineers had, etc.,. But the plot clearly indicates that the Engineers had planned 2000 years ago to go back and eradicate humanity, and that--in a strange way--earthlings actually have that particular alien free-for-all to thank (because if the resident aliens hadn't gotten loose and laid waste to the Engineers, they would've taken off for Earth as the lone awoken Engineer tried). So why would the Engineers have planned, at the birth of Christ, to go back and wipe us all out? It would've made more sense if, as the trailer erroneously hinted, that the Engineer they woke up did indeed find David an aberration and plan by his lonesome to go wipe out Earth.
Who cares. Why do we need to understand the motivations of an alien race. They are not human.
FYI, part of the original plot was they were mad that we crucified jesus, who was an engineer.
Anyway, the point is they decided their creation was shit. Or maybe they wanted to go use all the humans as incubators for a giant xenomorph army. There are lots of explanations you can come up with.