Of course the entire genre of cosmic horror is reactionary by its very nature, except when there is enough self-awareness to subvert or satirize the genre's reactionary logic, as in Verhoeven's Starship Troopers. But James Cameron's Aliens has no interest in self-critique, sharing a lot more in common with Heinlein's original novel.
Xenomorphs have only ever defended themselves from human colonizers invading their home, but we're expected to see them as evil, the scary other. The aliens must be bad because they pose a threat to us. Oh, and because they're ugly.
At least in the first Alien, the human crew members are sympathetic because they are merely surviving a situation they didn't want to be in, put in peril by a corporation sacrificing them for profit. Humans, not aliens, are the true villains of the film.
But in Aliens, our hero Ripley goes back to the moon with a special team of Colonial Space Marines to kick some alien ass. While this is ostensibly a mission to save a group of endangered colonists, Ripley has no interest in a search-and-rescue mission. She only agrees on the condition that they go there to kill every last Xenomorph.
Ripley is more than willing to exterminate an entire species to save one little white girl with blonde hair and blue eyes. In fact she still wants to genocide them even after safely escaping.
Ripley: I say we nuke the entire site from orbit, it's the only way to be sure
Burke: This is clearly an important species we're dealing with, and I don't think that we have the right to arbitrarily exterminate them.
Ripley: Wrong!
be sure of what, Ripley? you can just fucking leave. don't go back to the moon with all the Xenomorphs on it. seems pretty easy to me
now of course it turns out that Burke doesn't actually care about the Xenomorphs, he only wants to exploit them for profit. while this is keeping with the corporations=bad theme from the first movie, now we're supposed to think corporations are bad for... not wanting to do genocide? because of course no good person would be against murdering an entire species for no reason, only a villain would propose such a thing.
Now I'm not saying you can't enjoy Aliens, it deserves its status as one of the best action / sci-fi films of all time, and I'd argue these problematic reactionary themes actually make it more interesting and morally complex, giving us much to analyze and critique, elevating it above an average popcorn movie. Just please don't take it at face value.
Ripley is no longer the hero, even if she's portrayed as one. In Alien she is the scratched liberal, and in Aliens she is the fascist who bleeds. In a tragic turn, she has become the villain of the story. She reacts to her own trauma and loss of motherhood with mass murder, by killing another mother's babies right in front of her, and we're all supposed to clap and cheer, instead of asking why these humans are there in the first place.
The plot of Alien extremely heavily implies that the xenomorphs were not native to LV-426 and were being carried on the ship with the Space Jockey, who itself ran afoul of one of them. They're extremely unnatural in that a creature whose bodily fluids are a highly hazardous acid cannot fit into an established ecosystem at all and seem to be interested in nothing but consuming and reproducing endlessly
Also Burke was clearly cool with Weyland-Yutani doing experiments on human beings to weaponize the creatures given that's basically what that lab in the colony was doing
not sure how them being non-native makes any difference as to the moral justification for exterminating them. they were put there against their will and were there before us, anyway.
well, I suppose they'd just die off naturally without any other living organisms on LV-426 to eat or use as hosts, so in a way Ripley was putting them out of their misery (definitely not her motivation though lol)
Her motivation was "these things have killed hundreds of colonists and the Company wants to breed them and use them as weapons and fuck that"
Ripley wanted to kill em all before learning of the plan to weaponize them. She very explictly only agreed to the mission on the condition that they go there to kill every last xenomorph. At this point nobody knew how many colonists had died or if there were any survivors so her motivation was really about vengeance.
You could definitely argue that she suspected Weyland-Yutani of planning to weaponize the aliens. She certainly should have expected that based on the events of the first film. It does kind of beg the question why she'd agree to go then. If it was her plan all along to sabotage the mission then why tell Burke her true intentions?
Her motivation is she experienced her entire crew being killed by one of those things and she knew it was too dangerous to let out into the wider universe, so the thought of an entire colony of people being overtaken by these things is a tragedy beyond measure and one that could potentially spread to other colonies with similar results
They're space fire ants that are total assholes to everyone it's perfectly reasonable to want to call Space Dale Gribble
To be clear ALIENS does have a reactionary ethos but it mostly comes from the vilification of the military and the Space Vietnam narrative, but it's not really from the aliens. In Starship Troopers it's pretty clear the populace is being lied to about the bugs and the bug war; in ALIENS the xenos are a serious threat to human beings because they view us as prey and are very good at killing us as well as hiding inside of things, like rats bearing plague bacteria on cargo ships.