Pulling from the General Trek forum quotes
Yeah, Nerys and Richard are spot on with my general dislike of dystopia and horror. Horror is generally not my genre, though I do like classic monster stories like Frankenstein, like the Mummy, or Dracula. And I thoroughly enjoyed Jurassic Park. But, Alien is too frustrating because it's a nearly unstoppable monster with no rules or limits until one person defeats it temporarily.
I love nature and studying the ideas of alien life, but things that are just unstoppable are not interesting to me.
Actually the xenomorph was made to end the unstoppable monster trope. The xenomorph is indeed killable but its acidic blood makes it too dangerous to kill onboard a spaceship. Unless you lure it into an airlock.
In ALIENS the Colonial Marines have no problem killing a xenomorph outright with their Pulse Rifles and Smartguns. Its just the xenomorphs had numbers on their side and the nature of their hive structure allowed the xenomorphs extra cover from the marine's weapons. But put a xenomorph out in the open field, they are much easier to deal with.
List of xenomorph deaths.
ALIEN: The xenomorph Big Chap/Kane's Son is jettison out of the airlock from the Narcissus lifeboat. It is propelled from the engines and technically did survive the vacuum of space until being truly killed off in Alien Romulus
ALIENS: Several Xenomorph Warriors are killed directly by the Colonial Marines, their Sentry Guns, Ripley running one over with the Marine's APC, the stowaway xenomorph more than likely died in the dropship crash after killing its pilot. Ripley destroyed several Ovamorphs (xeno eggs) and facehuggers while rescuing Newt. And finally the Xenomorph Queen is blow out of the dropship bay, presumably to fall back into Lv-426's atmosphere to burn to death and since xenomorphs hate fire... you get the picture.
ALIEN 3: The Xenomorph Runner may seem unstoppable but due to the lack of firepower it was free to kill anything in its path. Until they trapped it and dumped molten lead on it. Yes it survived until the cold water sprinkled on its carapace which caused it to explode. Its clear the xenomorph didn't survive that.
Ripley kills herself along with the Queen Chestburster as they fall into a massive burner.
ALIEN RESURRECTION: Several clone xenomorphs are gun down by the protagonists. The cloned xenomorph queen is killed by the Newborn hybrid. The Newborn hybrid is killed by clone Ripley. The rest of the cloned xenomorphs die aboard the Auriga when it explodes in Earth's atmosphere.
ALIEN VS PREDATOR: Several Xenomorphs are killed by Yautja (Predators) in the backstory with just simple spears and their plasma casters. Several xenomorphs are single handily killed in the present by Scar (Predator protag) via Plasma caster, Shuriken, and Spear. Lex (the human protag) kills a xenomorph with a Spear and another with a laser gun.
ALIEN VS PREDATOR REQUIEM: Several xenomorphs are dispatched with 20th century era weapons. Wolf (Predator protag) kills xenomorphs with plasma caster, plasma shotgun, spear, blue dissolving solution, serrated whip, and shuriken. The rest the xenomorphs are nuked to death since there was alot of xenomorphs.
PROMETHEUS: This movie doesn't help my case since none of the xenomorph-related creatures ever died to unnatural causes. Hammerpede kills one guy then flees, never to be seen again. The deacon bursts out of the Engineer and never encounters the main characters because they already left the planet. However the Fifield Mutant does get killed after getting shot, set on fire, and ran over to death. The Engineer gets facehugged by the Trilobite.
ALIEN COVENANT: Xenomorph-related Neomorph gets shot to death by a human. The other Neomorph gets killed offscreen by the Praetomorph (David's attempt at making a Xenomorph). The first praetomorph gets crushed by a mechanical claw and the second one gets impaled by a forklift then sent plummeting towards Planet 4's atmosphere.
ALIEN ROMULUS: The Alien from the first movie survives only to be awakened, cause havok on the space station before getting killed permanently. Several cloned xenomorphs are killed by the protagonists via gunfire. The Offspring Hybrid is ripped apart by the planet's ring debris.
As you can see the Xenomorphs are definitely are not unstoppable. They can be killed.