It's not even an issue as it's not even in the film.
I have nothing against 'space zombie' Fifield. It's just what the film needed in the second act. Fifield is the main physical threat in the movie, if you think about it. The Engineer is an existential threat, but he seemed less interested in the survivors than wiping out human civilisation. The hammerpedes were not actively attacking people, either. The Trilobite was contained for most of the movie. So that leaves it up to Fifield as the main physical threat of the film.