I’m curious as to why the Xenomorphs from the Alien series tend to kill some of their hosts instead of infecting them with the face huggers? If they are to reproduce by using the hosts to carry out their life cycle then why kill them in some instances?
Because they are smart enough to know when it's time to try and breed, and when it's not safe.
- Alien 1: Alien on the ship-crew alerted to presence and capabilities. Unsafe to capture crew members to attempt breeding. Kill almost everyone and infect the survivor.
- Alien 2: Marines vs Aliens. No chance to infect new humans-until the stowaway at the end, and even then, the queen was discovered/killed.
- Alien 3: Aliens in the prison-dealing with violent inmates makes for a hard breeding process.
In any of these movies, it would have been virtually impossible to capture and infect, as opposed to stealth, eliminate, use remaining as they are no longer a threat.
This is just my opinion, But I believe the ALIENS facehugger were actually a distinct part of the lifecycle of the whole species. AND I also believe the whole species had a less than human intellect.
What made them so effective in their adult form was their natural weapons: Claws, Teeth, Acid Blood, Insanely strong, Highly resistant to injury
The immature facehugger was the stage that infected hosts. It did so instinctively and was fully non-sapient. It was the face-huggers that caused the Horror aspect of the whole movie to ratchet up several notches.
After seeing Prometheus, I suspect the entire species was constructed as a Genetically Engineered failsafe against the expansion of humanity. This being the case, it wouldn't make sense for the species to be perfect at it's job. If it was too good, then like any other parasite that is too good, it would simply kill off it's entire host species.
As I see it, the only method proven to survive an ALIENS incursion, is to be a highly aggressive species that at the same time is capable of extreme co-operation under stress and high danger, coupled with the ability as a group to determine flexible success outcomes.