It's a valid point, but there's a difference between doing an immoral thing to survive and doing immoral things to get home faster..
Sure, it's not as if I wanted to say their behaviour in killing the aliens to get home faster was justified in any way (it wasn't), just that their circumstances might have been far more desperate from the very start (being a much smaller ship) and they perhaps already had been pushed to some morally dubious acts before ever meeting these aliens, just in order to survive, making 'this final frontier' to cross a somewhat smaller step to them. Moral corruption often happens in small steps, after all.