20 Things You Didn't Know About The Xenomorph

Although Xenomorphs appear to be the ultimate killing machines, they actually serve a much higher purpose, and that is to grow their species. That's why, when given the chance, they would much prefer to subdue their victims and turn them into hosts for the next generation.

Only if an adversary is deemed too big of a threat will the creatures go into kill mode, which explains why they attacked the Marines but Newt was kept alive after being stolen away from Ripley in Aliens.