The Evolution of Sigourney Weaver in Alien

Ash is a character who got pushed so far that he's like, 'I'm not being terrorized anymore. I'm going to terrorize the monsters' The other big element is that the main character or characters eventually get so beaten down that they reject their victim status. That's one big element, the twisting of the knife. Emotionally twisting the knife to switch back and forth between being your loved one and being a monster. But then the question is, how do you make an alien without Sigourney Weaver? Do you just always have a character have that same arc?