The Xenomorphs would mercilessly kill and eat. The queen is a gigantic one, which lays eggs. These eggs hatch into "facehuggers", which cling onto a victim's face and, later, small aliens called Chestbursters would pop out of their chest, hence the name.
The blood of an Xenomorph is acidic, and they are covered in spikes and claws to kill prey. The Xenomorph queen must lay an egg containing a facehugger that then inpregnates a host in order to infest.