why would they behave as a human,
You've just reminded me of that great
Legend line "We are all animals my lady"
.
I always think back to
Alien, because there's a lot made of it being a version of a human. "Kane's son"; "It's like a man, it's big"; and there's even the Lambert death where Veronica Cartwright alludes to it "checking [her] out". So there's a lot of extra, and very disturbing, stuff there beyond it just being a ferocious beast.
There's also its body shape. You remember how the
Alien: Resurrection ones had hind legs even though they apparently came out of humans? Didn't make sense where it did in
Alien 3. They've lost that human aspect over time that made them really unsettling. Arguably even
Aliens, as great as it is, weakened the menace by having the more relatable structure of a queen, warriors and drones.
I liked
Prometheus as well, especially the Engineers.