Riddly Scott wanted a darker ending for the movie Alien !! At the end of the movie Scott wanted the Alien to bite off Ripley's head and then make the final log entry in her voice but 20 century fox didnt like the idea.
Good point there. I also like them the way they are - as just a normal species just doing what they do. Making them obvious villains would be disappointing and a bit forced; because then we are just trying to ignore the fact that we humans are just as bad or worse then them. Even the Ripley quote to Burke touches upon the issue slightly;
Ripley: 'You know, Burke, I don't know which species is worse. You don't see them f**king each other over for a goddamn percentage!'