David is not the engineer... it's pretty evident from all the tidbits and clues that have been released + the trailers + the named actors actually portraying the engineer in the film.
Besides, I don't buy the notion that the humans infected, 'become' someone else, engineers or whatever. They are still themselves with their identities intact, but something very terrifying happens to them, forcing them to make desperate choices. I think infected Fifield is tearing the people apart in the hangar because they essentially wanna kill him... Should he just allow them to?