David 8 in Prometheus didn't seem like he was behaving as he was advertised to do, in the original commercial. I think he was modified, or, like in Blade Runner, started to develop his own thought responses-- you know, love, hate, fear, envy?
That, or Weyland removed certain safeguards that would normally be in place to allow David 8 to be more flexible in carrying out Weyland's orders.
Or, David was underestimated by his own creators, being more intelligent and self-aware than anyone anticipated. After all, Weyland said David lacked a soul, but if his behavior was his own, wasn't programed, perhaps Weyland was wrong about David.
The cool thing about Prometheus is that it's never explained, or explicitly stated in the film, and any of these explanations is possible, given David's enigmatic and duplicitous nature. Beneath that comely and calm visage lurked a cunning menace.