What if it isn't a mutated crew-member?
What if the crew just thinks it is, but really it's an "alien creature" that instead of just gestating inside the crew member and sucking off nutrients (a la *the* Alien) or mutating the crew member, it totally consumes the crew member and, just like *the* Alien, gets some of its DNA from the crew member it has consumed so the general features are the same (bipedal, two arms, etc.). Inside the suit, how can you tell what's happening? *The* alien went from a chestburster to fully grown in 3-6 hours, something could grow inside the suit and completely replace the crew member in the same period of time or even faster. Whatever Ridley wants it to be, really.
The possibilities for just *what* the "Mutant space zombie" is really are endless, people are limiting themselves in trying to figure out what it is.
