This was arguably due to keeping very close to the themes of the movies. Remember in Aliens, when the marine's radar screen went nearly white with all the advancing aliens? Check. Remember the later scene, when it's implied one alien could take on the whole squad? On default settings, aliens can casually walk through a roomful of marines, limbs slashing wildly without even trying to target, and look back and see no movement. And then there's first-person skull-chomping mode...
The Marine campaign.
Let's set the scene. You are, of course, the only human in the area. You can see the bloodstains where other humans were, but at the moment you're alone. Most of the lights aren't working. You don't have a flashlight. You do have an infinte supply of signal flares, but only three can be active at a time, they don't last very long, and they don't really help, what with the flickering light and the sputtering hiss they make. There is no background music, no cues to listen for besides the sound of claws on metal. And there is, always, your helpful Motion Tracker:
And then, faintly, over the mechanical racket, you hear the claws. And the snarls. And the squeals. And you can't see where they're coming from, and by the time they show up on the tracker they've already moved, but you know they're close. But on the bright side, after playing this game, everything else seems a little less scary.
The fact that nothing happens happens throughout the first mission did not help. Besides the predator killing some of your fellow marines, also freaky. The occasional bleep, the goddamn cat scares (including the above-mentioned), 'Nothing Is Scarier'. The worst part is that at certain points, you can hear the bugs moving around.
On the subject of the second game, there's this lovely part where you play as a chestburster graphically chewing open a man's viscera and ribs. If you really want to see it, you can find it right here. Sleep well.
Not to mention the generous helping of Fridge Horror when you play the Alien and Predator campaigns and discover how horribly exposed you were the whole time - to the other two species you basically glow in the dark.
There's also Extinction, which is AVP as an RTS.
In Requiem there is a particularly bad scene involving a pregnant woman and a chestburster.
Just TRY to play the Marine's campaign at night.
In AvP 2's Alien campaign, the first level has you control a facehugger in search of a host. The second level starts in the host's torso.
The first game, as noted, was more focused on causing terror in the player, and it shows - near pitch black constantly, none of that cinematic music, more intense gore/death screams, a slower-refreshing motion tracker for the Marines, and immense single-player difficulty, especially as the Predator once you pass the first few levelsnote Granted, the Predator can take more damage and has a medicomp to heal himself while the Marine's health drains so quickly that he's practically a One-Hit-Point Wonder, but you couldn't restore your energy manually, and the Aliens just kept respawning on some levels..
The first level of the Marine campaign in AvP 2 is probably the most notable for inspiring paranoia and fear. You don't even fight anything until at least three-quarters of the way through the ridiculously long level—but you see and hear a lot of survivors getting picked off around you by Predators; one time, a boot falls between the pipes running along the ceiling and lands in front of you, accompanied by a lot of blood. All the while, you find Apocalyptic Logs about how the station got so messed up and what the survivors have been going through since.