Aliens and Monsters
"He got an e-mail from his brother that said that aliens and monsters were attacking his place!"
Of course, we can have alien protagonists and monstrous supporting characters; but the difference here is that, within the ethics of the shows that use them, it's okay to kill the specific threat-of-the-week version (which is usually a distinct species.) There is no need to deal with complicated intricacies of interstellar diplomacy to negotiate with aliens, consider ethics of advancing mankind via genetic engineering when dealing with mutants, and listen to a vampire's tragic past to understand him better. This time, there are no long term negative consequences to deal with either using what humanity does best.
In short, this trope is for a specific example of Black and White Morality when a non-human antagonist (and, likely, his entire species) is Exclusively Evil with a shallow, handwaved, or Played for Laughs justification. Different from Aliens Are Bastards, in which the reasons for hostility can be elaborate and well-explained, and often the subject of much debate and comparison to conflicts among humans.
- Doctor Who is king of this trope, to the point where, eventually, monsters would make their way into stories in which it has no place ('The Caves of Androzani', for example, is a thriller about interplanetary politics and the ruthlessness of unfettered business practices against a weak military and political sector, with a strong cast of interesting villains in its own right...which features an unconvincing monster in a cave because, well, it's Doctor Who, innit?). Ironically, the show was supposed to be a historical edutainment program, until the Daleks showed up in the second story and royally EXTERMINATED that idea.
- Buffy the Vampire Slayer had many demons of the week. Remember that one that Buffy had to beat the crap out of, while Giles and Willow cast a spell on it?
- Inverted in an episode of Angel. Cordelia has a vision about a demon and Angel proceeds to find and kill it as usual. Immediately afterwards, he finds out that the demon was actually on a mission from the Powers That Be and he should have been helping it.
- The original The Outer Limits was specifically designed to have one "bear" every week, the producers' code term for aliens or monsters.
- Special Unit 2 is based on this trope, combined with All Myths Are True (except vampires).
- The Reavers serve this purpose on Firefly, but the trope is actually more often subverted than not, as there are no aliens and the characters are more likely to find themselves up against other humans than anything.
- Let's not forget the antagonists in every episode of Power Rangers ever.
- Tech Infantry has vampires, mages, werewolves, weretigers, wererhinos, alien bugs, alien lions that fly, and few of them are as scary as some of the normal humans.
- Paranoia has included suggestions for alien invasions alongside the Giant Mutant Radioactive Cockroaches; of course, any mission in Paranoia is just a Framing Device to let the PCs betray each other, so...
- Monsterpocalypse: there are always 50 foot tall monsters, like invading aliens, hungry dinosaurs, giant Gorillas, and so on...
- Sluggy Freelance has plenty of aliens, demons, and vampires the cast kill without a qualm, even though they also count several aliens, demons, and vampires among their friends. Aylee called Torg out on this when he spent his time in another dimension killing the monstrous residents in sadistic and hilarious ways.