All together now: "They mostly come out at night... mostly."
One of the themes in James Cameron's Aliens is meritocracy, and the way extreme circumstances can enforce it. The ascending chaos which dominates the story functions as a kind of sorting mechanism, through which false hierarchies are upended and true character is sifted down to its rightful place. The officious, incompetent Lieutenant Gorman is effectively removed from command after his paralysis at the atmosphere-processor battle reveals how unfit he is; after he regains consciousness in mid-Act II he meekly cedes authority to Ripley and Hicks, and has little else to do but redeem himself in the eyes of Vasquez by blowing them both up rather than submit to capture by the aliens.
Hudson, initially one of the main voices of military bravado among the Marines, is first reduced to impotent whining by the series of disasters befalling the expedition, and then finds his inner steel after a couple of terse pep talks from Ripley. He dies, but -- like Gorman -- has redeemed himself.
Alongside a hierarchy of competence is one of morality, in which yuppie-scum Burke, who at first shows promising signs of okay-guy-ness (he backs Ripley's bold move to commandeer the APC and rescue the trapped soldiers, and later gamely hauls supplies to assist in waiting out the alien siege), rapidly declines in the second act -- from shifty dissembler to out-and-out murderer. He gets an alien in the face as a reward, but Cameron is not so simplistic as to kill only the evil and useless. Apone, Drake, Frost and Dietrich -- all good, serious soldiers -- are quickly iced in the first major battle, while (in one of my favorite moments) ultra-cool dropship pilot Ferro shows a moment of steely decisiveness (going for her gun rather than gaping in terror) before an alien does her in. Cameron keeps us guessing by at first obscuring, and only later illuminating, the relationship between moral/professional stature and survival. More and more, the bad or stupid meet a sticky end, while the redeemed are granted at least a Good Death.
The power structure at the beginning is a sort of hypermasculine, inflexible, and overconfident military hierarchy, but after the sorting mechanism has done its work, what we are left with is a nuclear family with Ripley as the mother, Newt the child, and Hicks the father. (It's less clear where Bishop fits in, but significantly, he has no military rank. Asexual and unfailingly courteous, he comes across by the end as a sort of helpful butler.) If our emotional response to the first power structure is jarring and discordant (note the loud, obnoxious behavior of the soldiers, which initially alienates Ripley), a sense of harmony is restored in the final scene of Ripley and her adoptive family going to sleep, appropriately scored by James Horner with gentle woodwinds.
At the film's midpoint, after the drop ship has crashed and apparently stranded everyone, Cameron drives home the inversion of authority with a key shot in which Ripley and Newt are placed in the foreground, gazing with steely determination into an uncertain future, while behind them Hudson and Burke uselessly bicker amid the flaming wreckage of once-intimidating military hardware. From here on in, Ripley is ascendant and all the Good characters will follow her lead.