I don't know what to think, if you look closely at the trailer, when they show the guy over the waterfall, it looks like his head starts to melt, and if I remember correctly, when the first leaked images appeared, there was a screenshot with David, and he appeared to be bald, and crying. So, maybe he did became the Engineer, as he seems to be very happy with what he found on the alien ship. He is always smiling, and we see him floating into the universe map. I think David will be the bad guy in this movie.