...was the unswavering focus on who the Enemy really was.

I mean, when you get down to it, the Aliens aren't actually Evil. Bloodthirsty predators, yes, but evil, no. They're just doing what they're good at. It's humanity, ultimately, that is the betrayer in each film - turning its own over to the Alien hunger in order to gain profit.

After all, it's why we cheer in Aliens when that corporate schmuck gets the Jaws 'o Death from one of the critters...