I don't see what the fuss is about.
It's either created by another lifeform, on purpose or by accident.
Or it's made by nature. And then it's just a space bug.
So an artificial lifeform in the shape of an android turned mad scientist/artist being responsible, ain't that bad imo.
Ofcourse the Engineers would've made more sense. But then it might just have been a less complicated black goo creation.
Too bad they didn't incorporate Advent, more drawings and the stories behind them in the movie.
I find the whole creation process mentioned in the writings pretty cool. Seeing how David took qualities from different species, combined them, studied the results, tweaked them,...