I was thinking they may be the real creators' first species. I was mainly drawing on Milton's Paradise Lost as an analogy: The Titans (or Watchers) refused to accept Humanity as their brothers, just like Lucifer refused to bow to Adam, precipitating not only a civil war but a rebellion against the creators, as well as a desire to exterminate the "little brother."
A sibling rivalry of sorts is alluded to in the movie between the synthetic David and Miss Vickers - I figured it might be a reflection of the larger plot behind the movie.
