The aliens in "Alien" (and it sequels) are sentient.
Sure, they don't talk, either to the humans or to each other. But that doesn't mean they don't communicate. Maybe they "speak" in ultra-sonic frequencies (those big, dome-like foreheads look suspiciously like the skulls of dolphins, used for echolocation) or use pheromones or telepathy or some sort of other sci-fi communications. And yes, they are astonishingly single-minded predators, and you'd think that an intelligent species would display some sort of ethical structure to their behavior. But intelligence doesn't necessarily imply culture, especially when (as is definitely the case in the first movie) each new alien has to create its own civilization from scratch wherever it's born.
But if you watch the movies under the working assumption that they're actually very smart, if uncivilized, a lot of things make sense. The alien in the first movie doesn't just wind up in the escape pod by chance, it heard the announcements and decided to get the heck out of Dodge. The aliens in the second movie display excellent strategy and tactics, placing their nest in a spot that neutralizes their enemies' most powerful weapons while allowing them free use of their own. And the alien in the third movie practically runs a textbook guerrilla warfare campaign against its opponents right up until the end.
So the next time you watch the "Alien" flicks, don't think of them as mindless, voracious killing machines. Think of them as extremely intelligent, voracious killing machines. Somehow, that makes them even scarier, doesn't it?