By A3 the creature was basically reduced to a mindless slasher monster, because that's apparently what the producers and movie studious thought was the basics of the success.
For that I love Prometheus' effort of doing something deeper. It could just have been better by staying away from the slasher/horror movie cliche formula. For one thing... I would have loved to hear more about what made Shaw and Holloway come to the conclusion that we were engineered by the alien creatures pointing towards the stars - it just skipped over that to go to the boring horror crap with stupid characters. I still would have liked at least some discussion amongst the characters about what they were discovering... what they were traveling towards... SOMETHING other than mindless "hey the robot's bad, no he's not, yes he is" and "ooh, I'm so stupid, I'll pet a penis hissing at me". That's what crappy movies do... skip the interesting stuff to go to the "action" and "stupid deaths".
In all honesty... showing no "living" aliens at all in Prometheus could even have been better.
- going to the moon, finding the silos, finding the decayed corpses, finding the head, finding that it looks human, finding the ampules, taking one back on board for examination, the infection spreading, the people turning on each other, David discovering that the engineers had targeted Earth for extermination/advanced evolution through the black goop, but that they were stopped by an outbreak. David saving a few of the people (or just Shaw) and takes them into one of the derelicts, them deciding to travel to the engineer homeworld (due to orders from Weyland who would still be in stasis).
But at least have the characters discuss, argue, react to what they are finding.