Most of the movie is spent with the characters being hunted by the titular Alien, being found and then killed, which while not quite stalking, could be potentially triggering
Not necessarily abuse, but Jones’ cage is flung by the alien and then thrown around a bit by Ripley during her escape. Can make cat owners uncomfortable.
In the third act of the film Ripley makes it a point to recover Jones the Cat to evacuate him with her before the ship self-destructs. There's a moment of suspense when she has to abandon him when she catches sight of the monster but ultimately she gets herself and Jones off the ship. He's sleeping peacefully in her lap in the final scene.
The crew's attempts to remove an alien parasite latched onto a crew member's face involve a number of close-ups of the creature, showing it in full detail. The prop, made of various seafood bits among other things, is realistic-looking and may be unnerving for some.
There is no literal sexual assault, but there are enough horrific metaphorical allusions to it that a sexual assault survivor would probably have their trauma triggered. Phallic objects being forcibly shoved down throats is a recurring element.
Dallas was captured by the alien and asks her to kill him towards the end of the movie, she uses the flamethrower to do it. You do not see him get burned but you know it happens
There are multiple violent murders including an infamous “chestburster” scene in which an alien rips apart a character’s ribcage while leaving his body. There is a lot of blood.
Well gee, the cover's only used as the emblem for jumpscares... In all seriousness, the Facehugger leaps from its egg violently, and Davis is attacked by the Alien suddenly when searching the airvents.
As spoiler free as possible: a character attacks Ripley. At first his sweat looks weirdly white. As he is trying to suffocate her other characters come in to stop him and hit him. He starts to spew white fluid of his mouth a little bit. Later when the crew wakes him back up the same white fluid leaks out of his mouth before he is able to talk.
People awaken from, & enter the hibernation pods in their underwear. No sexy lingerie, undershirts, boxers, Sigourney Weaver at end in basic bikini pants.