Fig 1: Living JelloMusic for the Article
There is an invisible value placed on the existence of non-human characters in fiction, compared to the value of the life of a human. Destroying one may or may not be the same thing as destroying a human.
• Obviously nonliving things like inanimate objects do not figure into this at all... Unless you're in an Everything Talks situation where the objects are given names, faces, personalities, and so on. And especially if, in the case of the broken-down cars in The Brave Little Toaster and Jessie from Toy Story 2, they sing sad, sad songs about the day their owners threw them away.
• Robots and Artificial Intelligence stories examine this quite a lot in their plots, possibly because of the writings of Isaac Asimov. Good robots and other Mechanical Lifeforms are considered people most of the time. Harming one is generally the karmic equivalent of harming a human the same way — except that it is easier to show them getting hurt, which gets awkward. Mecha-goons and bad robots almost always have a very low value in this regard, even if they demonstrate obvious personalities, emotions, and humanlike intelligence. Regardless, robots are the most frequent victims of the "How Did You Know That Goon Wasn't Human?" "I Didn't!" conversations. It's Just a Machine, after all. It probably helps that when a robot dies We Can Rebuild Him more easily than bring back a human (which is a source of superiority as well: human life is more complicated, probably because robots are always written as not having souls even if they are sentient), making them more expendable.
• Undead beings like skeletons, zombies, ghouls, and victims of certain strains of The Virus do not blip at all in this value (despite still being Homo Sapiens). There's hardly any controversy about it either, probably because they're trying to harm you. In fact, destroying one is seen as only helping along a natural process. There are some exceptions in the very, very rare works where the zombies are not entirely mindless and retain a bit more personality and/or self-control.
• Vampires, while they are technically among the undead, have variable ranges simply because they usually have more personality. Most characters can destroy them anyway. The idea here, as well as with the other undead mentioned above, may be "Well, technically, they're already dead, so it's okay! And anyway, Vampires are Always Evil!" Expect that last detail to make things awkward in fiction where there are good vampires, or vampires who aren't evil, just hungry, operating in the same world. It should also be mentioned that a typical way for a Vampire to are destroyed (turning to dust/ash) also means that it's a lot easier to show a Vampire dying or being destroyed onscreen. Considering that the original way of smiting a vampire was far more complicated and involved (i.e.: you had to turn it to ash the hard way), it should probably be the other way around: that Vampires started dying that way because it was safer to show on TV.
◦ Special mention must be made of villain who want to be human as they are often on the same level as vampires on this scale (and more than a few vampires have been wanting to become human). These are characters who would be considered normal people were it not for a few very strange differences. The troubling part is this: even though they often look like normal people, even if they go on and on about how they wish they were normal people (and they often gain the audience's sympathy in the process), none of the heroes seem to take any of this into consideration and dispatch them with clean consciences. Eerily, some fiction in which villain with a desire to become human appear even acknowledges how twisted this is — and let the good guys blithely destroy then off anyway.
• Plants, protists, fungi, bacteria, and so on and so forth do not count on this scale. Except sometimes when tropes like Plant Aliens, Planimal, and Plant People. Or if the organism is a member of an endangered species. Or if you're talking about destroying an entire forest, since that's on such a large scale and since there are animals in the forest that could be destroyed or get their habitats destroyed.
• Monsters Of The Week, Giant Monsters and Bugs are generally treated as huge pests and exterminated as such without much controversy, typically in self defense. There are some exceptions. If you are a monster, the more you resemble a more conventional specimen of the creature you are based upon, the fewer people you directly harm, and (most importantly) the more personality you have, the better your chances are for surviving. Some human or other will recognize that you are merely misunderstood and may try to help you. Of course, if you eat that human, you're pretty much doomed.
• If the Big Bad is revealed to be non-human as a surprise or assuming his monstrous true form, it usually makes it OK to destroy them them if it wasn't before.
• Human-like Aliens rarely have this problem - as their actors are obviously human, it is easy to transfer the value (this is largely why the trope persists even into the modern, CG-heavy era). Humanoid Animals and Half Human Hybrids tend to get the same protection as a normal human... but it depends on how humanlike they are. If they take up a form that isn't bipedal, rely on their instincts too much, or otherwise start toward the Talking Animal side of things, they can quickly reach the level of monsters-of-the-week.
• As far as other fantastic races, it often seems that the morality of destroying the race depends on how much they resemble humans either culturally or physically. Dwarves, elves, gnomes and halflings all look relatively human, and so destroying them is bad, but the bestial-looking orcs, goblins and trolls are evil and should be destroyed. Other races who obviously are not human, but possess cultural traits such as music or clothing styles that the human audience can easily recognize or identify with, are also given preferential treatment over whatever evil races exist.
• Clones or parallel universe duplicates are often considered expendable, even if they absolutely are biologically human and independent individuals with unique personalities. Restoring an AI from a backup copy is often treated like a near death experence. This is all provided at least one "instance" of each character survives.