Depending on the author, Batman varies greatly in this regard. However, he is shown many times to consider all sentient life sacred. An example would be where he believes he accidentally killed Judge Death in the Batman-Judge DreddCrossover, despite Death being an undead killing machine that just killed three people in front of him.
Batman: I didn't mean to kill him! Random police officer: It was a monster! It just killed (policeman's name)! Batman: That doesn't mean I had the right to take its life!
Not that this mattered; turns out it takes more than being impaled to kill Death anyway.
Batman's no killing clause varies depending on the author. He has no problem killing undead in the above-mentioned Batman Vs Dracula, and in Batman/Aliens he goes ahead and jams a Bat-grenade down a Xenomorph's throat.
In a late-1990's Elseworld-type story, Batman teams with Tarzan, and finds his new ally has no problem casually tossing a mook off a cliff. When Tarzan risks his life (and the mission) to save a lion, Bats reads him the riot act. Tarzan's response: "The man was my enemy - the animal is my friend."
Their twist on the matter involved an alien shapeshifter escaping to the Big Meg to get out of slavery. Dredd, who wants to arrest the shapeshifter for murder, is partnered with an alien administrator who wants to return it to its owners. The shapeshifter is finally left with the decision to either stay in Megacity One, where he counts as a sentient being and is therefore subject to the city's comically strict justice and faces a long prison term, or return to Alientown, where, legally counting as property, he can not be held culpable for his actions, but will remain a slave.
Robots are sentient and exactly like humans, but are still a slave race and abused. Dredd himself is responsible for destroying a robot revolution and sending everyone back to slavery. (Usually the writing is on the side of the robots, as is reader sympathy.) Note that Dredd himself is a robot-rights supporter; the only reason he shut down that particular revolution is because the robot leading the revolution was clearly evil.
XTNCT: Humans use genetically engineered animals to fight their wars for them, and are able to discard or replace them just as easily.
Probably the most interesting exploration of the zombie issue is the web-turned-actual comic Dead Eyes Open where people start coming back to life as zombies — but retain all of their memories and personality. They don't even start eating human flesh. The ramifications of intelligent living dead are the focus for the rest of the story.
In Runaways, during the Civil WarCrossover, the Cape Killer unit is shown to actively rate an enemy's worth based on how much the news-viewing public might object. Minors are treated with non-lethal force, adult humans with moderate force, and with artificial beings like Victor, it is permissible to use full lethal force. Aliens have no legal standing in the US and do not generate any sympathy from news-viewing audiences, so it is considered the same as hunting an animal.
In another story, Darkhawk said: "You're not made of flesh, Ultron, which means I don't have to play nice." Of course Ultron is a complete scumbag who long ago crossed the Moral Event Horizon so this may have more to do with that than this trope.
The Bizarros in Silver Age stories were another example. Made of "non-living matter", having them killed off was often a source for comedy; they even made a meteor plunge into one of their cities and kill a lot of Bizarros, on the grounds that Bizarros do things backwards so they want to maximize casualties. This was played as a pure joke.
Another classic Superman story from #314 (back in 1977): Superman is faced with a dangerous alien Jevik, who he intends to destroy. When questioned about how he can kill when he has a code against killing, he replies that Jevik is not really alive. When Jevik's heart begins to beat, Superman says that somehow he's come alive, and Superman can't kill him. Apparently for Superman, if you don't have a heartbeat he can do anything he wants to you; merely being able to walk, talk, and act on your own doesn't qualify.
He made a few exceptions for drastically nonhuman things, like the Eldritch Abomination Urko the Terrible; and when Brainiac underwent his first major upgrade (to his Skele Bot form) and became far more ruthless in killing innocents, Supes said he'd be willing to destroy him since he was Just a Machine. On the other hand, the readership revolted when Superboy destroyed the very first Bizarro, and so the character was recreated by Applied Phlebotinum not long after.
Although that backstory has long been retconned out, "World of Krypton (Volume 2)" established that this sort of thing was the reason for Krypton's cold and sterile world: at one time, Kryptonians grew clones of themselves to extend their lives and, pretty soon, people started to get uppity about it, going into a major war that would lead to the planet's ultimate destruction.
In Red Daughter of Krypton, Supergirl destroys Worldkiller-1, an artificially-created parasitic sentient biological weapon, reasoning that it isn't a true living being but a mockery of one.
Supergirl: I know who I am, and I know what you are— A mockery of life. This is not murder. It is the end of a terrible mistake.
In the reviled Superman: At Earth's End, Superman uses "you're just an android, I AM A MAN!!" as justification when he punches Ben Boxer's guts out. There are many things wrong with this, least among them the fact that Ben Boxer is about as close to human as you can be, with emotions, a personality, and brothers. Oh, and intestines. This guy has relatives and can poop, man, that doesn't sound all that inhuman to me. Superman then condescends to him further, claiming that Ben is "only doing what your creators programmed you to do"... even though Ben actually has free will and makes his own decisions.
Parademons in New Gods serve as Darkseid's evil minions bred as modified clones from Apokoliptian citizens or recruited among the planet's most sociopathic denizens. Their sentience depends on the writer, as most are unquestionably loyal to Darkseid and do as they told, though there are exceptions like Mike and 3G4 who develop minds of their own. Most of the time, they are mowed down by superheroes without impunity (such as the DCAU, where even Superman vaporizes them without second thought). That said, there are times where the opposite happens when in Injustice: Gods Among Us, Superman unleashes his full power to eliminate Parademons all around the world at once and he is chewed out by Batman for taking so many lives, though in turn he gets called out by his own allies for criticizing Superman for doing what needed to be done.
Ghost Rider, at least in his 90s incarnation, did not kill even the most inhuman of humans, to the point where his apparent destruction of a ninja in one issue was retconned into that single particular ninja actually being a robot. However, he was quite happy to maim and slaughter demons and other Always Chaotic Evil beings, in one instance tying the photosensitive pseudovampire Blackout to the spire of the Empire State Building and letting him die a horrific burning death as the sun came up. Blackout didn't actually die and popped up on the Raft about fifteen years later, but Ghost Rider had no way of knowing that.
Maybe now it does, but historically that hasn't been the case; among other examples, they originally killed Brainy's malevolent AI creation, Computo, without a qualm, and in the next incarnation tried to do it again, although Superman was the one who struck the death blow that time. Then there was a later arc about an invading machine race which raised the issue of the rights of AIs in the Legion's society (namely, that they didn't have any); at the end the Legion's leader pointed out that they'd been killing the machines all through the storyline, and wondered if this was a violation of their code.
In one set of issues they actually published the Legion Constitution, which states that any Legionnaire who uses lethal force against a sapient being must be expelled, unless the act was in the final extremity of self-defence, or a provable only alternative to the deaths of other sapient beings. In the case of the original Computo story, his destruction was very definitely a "provable only alternative."
That's a later provision of the Constitution; at the time of the Computo story the Legion did not allow the use of lethal force under any circumstances, however justified...but no one blinked at using it on Computo.
In The Sandman, Richard Madoc justifies his abuse of Calliope to himself because she's not human; however, this is clearly portrayed as absolutely immoral.
In the 1980s, however, he was retconned to contain an alien Energy Being called the Tornado Champion.
In Hellboy: Conqueror Worm, Director Manning reveals to Hellboy that the BPRD upper brass decided to install a bomb in Roger the Homunculus, as a "fail-safe" to prevent Roger from endangering the lives of agents in the future; their explicit reasoning is that Roger is expendable because he's not human. When Manning gives Hellboy the detonator, HB is not pleased: "You know, I'm not human, either, remember? When are you guys gonna put a bomb on me?" Then, when an opportunity arises for Roger to kill the Worm by blowing himself up, he's perfectly willing to do so, and it's Hellboy who insists they find another way. By the end, Director Manning comes around to HB's point of view, but Hellboy is so ticked by the entire incident (and a few other factors) that he quits the BPRD.
The same story also contains the phrase:
To be other than human does not necessarily mean to be less.
Subverted in the original Fawcett and DC pre-CrisisShazam Captain Marvel stories. One of the odder characters is Mr. Talky Tawny, a talking tiger who is taught English and chooses to live with Humanity. He's well dressed and has excellent manners. Captain Marvel made sure that Mr. Tawny is treated as nothing less than a full citizen of the society he chooses to live in, which comes into play when Tawny is on trial for... mauling someone. (Incidentally, the pre-Flashpoint version of Mr. Tawny in the regular continuity is a magic stuffed toy brought to life, and the Shazam! The Monster Society of Evil version is a shapeshifter whose favorite form is a tiger.)
Jubilee was a particularly extreme example. Normally she's so firmly opposed to killing that she once abandoned an escape attempt — effectively giving herself up for another round of torture — in order to perform CPR on a random mook she had injured. Then during Marvel's Secret Invasion crossover, she was killing Skrulls without even blinking.
It was pointed out in New Warriors by that Jubilee's personality had radically shifted. Never got around to why before that one ended though.
The X-Men have also been on the receiving end of this. Since they belong to the mutant species Homo superior, they are technically nonhuman (though closely related) hominids... And many of the anti-mutant villains in the setting consider them exactly that, treating them about as considerately as they would human-like but evil and dangerous aliens or vampires.
Speaking of Skrulls in Secret Invasion every hero killed Skrulls by the truck loads without blinking even when there were beaten and on the run, even POWs eating for their lives (ok the only ones that kill those are HAMMER) it makes you feel real sorry for the Skrulls (even if these ones were religious fanatics who odds are would have wiped out the human race if it wasn't for their Queen.)
Heavily analyzed in Earth X, not so much with the mutated population of the world (they're all people), as with X-51 (Aaron Stack the Machine Man), who suffers from Pinocchio Syndrome. At one point, Uatu actually inverts the scale since he believes the Celestials are so far above ordinary organic life that we might as well be bacteria, telling X-51 that he is superior to the artificially-created organic being Woodgodbecause Woodgod is a biological lifeform—a "beast". Aaron never buys into this argument, though.
But he had adopted this viewpoint when he appeared in Nextwave. The official explanation is that he suffered an emotional breakdown after getting taken away by the Celestials and then seemingly rejected by them. His depression manifested as a general misanthropy (literal misanthropy: a dislike of humans).
Examined in ElfQuest. It's an unwritten rule that elves, especially the Wolfriders, Don't Kill Other Elves. When finally one elf has to choose between killing his enemy and losing his son, it's appropriately traumatic for the character when he decides to shoot. However, the Wolfriders consider themselves part of the forest, and as a result they hunt, kill, eat raw meat, return their dead to the earth, and never interfere when one of their wolves is cast out from the pack (which essentially means a lonely death). The Sunfolk, who live as oasis farmers, take it one step further and lived as vegetarians for close to 10000 years before a lack of rain forced them to take up hunting. One character who particularly fits the trope is the Wolfrider chief Mantricker, who enjoys hunting humans, but would never kill one. In an unfortunate case of a localisation entirely missing the point, his name in the Dutch version roughly translates to "Humankiller".
Concerning the interference with the pack, Cutter, living comfortably with the Sunfolk, is able to take care of his aged wolf companion in a kind of "retirement" after he is cast out.
in Ode to Kirihito by Osamu Tezuka, a newly discovered, fatal disease caused by exposure to high levels of mine runoff called Monmow's disease causes people to change into doglike mutants before dying within about a month. Dr. Osanai Kirihito is assigned to medical research in the African village where the disease originated by his boss in order to gather data on the disease.
Something similar has popped up in, of all places, the past few years of Witchblade; Sara Pezzini has read a monster its rights at least twice. The monster responds by attacking and she gets to kill it anyway.
Subverted in an issue of the Superfriends comics (70's version): the heroes rescue a beautiful woman from what appears to be a collection of movie monsters (including a werewolf and a mummy!) It turned out however, that she was actually a space criminal, and the monsters were- alien superheroes that just happened to look like our movie monsters!
Yoko Tsuno plays this straight and averts this as the plot demands. The insectoid Titans are generally treated with respect, for instance, since they do appear to have some human-like qualities. The antimatter-eating alien from The Time Spiral is considered to be "gross" though when it turns out to look like a jellyfish—and its fate therefore is simply to be killed without mercy.
Spider-Man nearly did this in an issue of Ghost Rider. He was facing the vampire queen, Lilith and a host of the undead. He attempted to use Johnny Blaze's gun on them, explaining that "They're already dead". In team ups with Blade, he has also not seemed to care much about vampires getting killed. This is the same guy who will box your ears if you try to kill Carnage, a super-powered serial killer.
There was a 70's comic in which the Avengers were blowing up alien ships sent by Thanos. Presumably, there were aliens inside. This is odd considering the Avengers had a strict no-killing policy at the time. What makes this even more odd is that many team members have been androids, aliens, or otherwise non-human.
This was subverted in the Operation Galactic Storm arc, however. Captain America and other Avengers were opposed to destroying the Kree Supreme Intelligence, a sentient super computer, because they considered it alive and thus its life was sacred. Iron Man and other Avengers disagreed and destroyed it in order to save the galaxy.
Depending on the Writer, the Supremor is a cyborg and therefore partly biological. It's a gestalt made from the brains of the greatest geniuses of the Kree race (though whether it literally contains some part of those physical brains, or just digital copies of their memories, is not consistently clear). Hence the big blobby biological-looking face it uses as its avatar.
It's long been assumed that Aquaman doesn't eat fish or would be offended by others eating fish. As of the DC New 52 relaunch, Aquaman shocks a restaurant full of patrons by ordering fish. As a telepath he knows the fish he's ordering have very low order intelligence.
While he doesn't get bent about people eating fish, he however doesn't tolerate rampant killing of sea life.
Fridge Brilliance. He and his people are water dwelling, they have to eat SOMETHING.
This quote also sums up his feelings about taking a life.
Aquaman: Do not assume I'm governed by some code against killing, or fear of legal consequences. No one will ever find your body.
This trope leads to massive Moral Dissonance in Teen Titans issue 100. Superboy-Prime attacks Titans Tower in an attempt to kill Superboy (Conner) and brings along a handful of Superboy clones grown from Conner's DNA. Conner brings out his emergency Kryptonite and two Titans without a no killing code, Ravager and Robin, kill the clones by stabbing them through the heart with a Kryptonite spike. Once Prime is taken down (and bearing in mind he's the most powerful and evil of any of the villains present by a mile) the same two suggest finishing him off. They're told "that would be murder" and "we're not killers". But killing the clones was apparently okay. And just to make matters worse, Conner is himself a clone. He even mentions that he started off as a "blank slate" like the other Superboy clones.
In the Avengers Assemble Annual, The Vision calls out Tony Stark and Hank Pym for leaving his disassembled body in a warehouse after he was ripped in half by She-Hulk. Though they were both preoccupied by various crises (and one of them was a Skrull at the time), that doesn't really change the fact they left one of their teammates ripped up in a warehouse. Even if they thought he was gone forever, didn't he at least warrant a proper burial?
In an issue of Young Avengers, Patriot and Hawkeye II teamed up with the Winter Soldier to fight a MODOC squad. Winter Soldier killed several of the cyborgs with his handgun, which prompted Hawkeye to ask if they were human. Winter Soldier explained that the men were essentially turned into "human robots" by AIM, and that the process was irreversible. He then argued that he did them a favor by killing them.
There was a series called Sentinel, which as the name implied, was about a boy named Juston Seyfert and his "pal," a reprogrammed Sentinel. In Avengers Academy, Emma Frost argued that the Sentinel's programming and memories should be wiped to make it less dangerous, which Juston claimed was essentially murder. Then after the Sentinel was destroyed while protecting Juston during Avengers Arena, he fell into a Heroic B.S.O.D. moment and stated that he no longer had any reason to live, since his best friend was now dead.
Completely and utterly defied in the Invincible spin-off Guarding the Globe. When Japandroid sacrifices herself to stop a global parasite infection, her death is treated exactly the same as if she was human. No one even mentions that she was a robot.
An interesting case is with Hulkling in Young Avengers. He isn't usually treated any differently than any other hero, despite his status as an alien hybrid being well known. However, his own treatment of people of his own race is very much filled with this trope. During the story arc when he first discovers his heritage, he at first reasons that he can't be a Skrull, and that his mother can't be a Skrull, because they're both so ordinary as far as humans go. Later, when he mentions wanting the Kree and Skrull forces to stop fighting, he makes a comment about how the Skrulls are "my people," with Billy, his boyfriend, responding with "They're not your people. They're not even people." Wolverine even gives justification as to why it's okay to use lethal force against Skrulls in this story, claiming that "They're Skrulls. They'll grow back. Eventually." In later story lines, despite having accepted his heritage, it's still obvious that everyone still considers him a human, and that he considers himself a human, and he keeps treating the Skrulls as an alien species that he can't relate to, which sometimes makes him come across like a Boomerang Bigot.
Only "Red" thinks of The New 52 version of Superboy (who is half-alien and a clone) as a human. (Although Rose may have a soft spot for him, too.)
In Iron Man "Fatal Frontier", Tony starts thinking this way when it comes to robots and clones. He is eventually called out on it by a reporter. It turns out that this is a symptom of phlogistone poisoning, which corrupts a person's very soul.
Death's Head discusses the rights of sentient robots and heroic ethics with Tony Stark while they are teamed up.
"You’re not one of those guys who has a code against killing 'except for robots'? I hate those krypto-fascists."
Briefly discussed in the Astro City story, "The Eagle and the Mountain". When Samaritan is disturbed at Infidel's use of female homuculi (non-sapient mindless apparitions) for his servants, Infidel asks him if he would've been disturbed if they were robots instead.
In Transformers: More than Meets the Eye, there's an entire test used to determine this, called the Ambus Test. Essentially it tests robots to see if they're actually sentient (in which case they have full rights) or simply highly advanced AI pretending to be sentient (in which case they presumably have no rights). The comic also features an inversion; the Decepticons are shown to have been fairly bigoted during the Great War, believing organic beings like humans to be non-sentient or simply unworthy of life; in other words, robots judged humans as not being equal.
Tech Jacket: Zack shows no remorse for killing hundreds of Kresh (gas based life forms) but is visibly upset after killing two human looking aliens in self defence.
Ultimate Marvel:Played With at one point during the Ultimate Galactus trilogy. Hawkeye has no problem killing several Kree in an ambush, something the Kree Captain Marh-Vell takes personally, even if he's a traitor.
Hawkeye: They die like people.
Marh: They are people. I'm one of them.
Hawkeye: Then you'll die like people too.
Archie Comics' Sonic the Hedgehog explored this concerning Nicole the Holo-Lynx with the Sonic Universe storyline "Spark of Life". Dr. Ellidy, Nicole's accidental creator, is very put off around Nicole due to the fact that Nicole was actually his daughter, Nikki, who died during an attempt to save her life by digitizing her. This attitude concerns Sally, who even asks Big the Cat his thoughts on Nicole, who likes her just because she's nice. When the digital lifeform Phage causes trouble in Ellidy's systems and Nicole's in danger, Sally opts to be digitized herself to save her friend. When Ellidy tries to stop Sally from doing it because she's a princess risking her life, she essentially puts him in his place by telling him to never suggest that Nicole isn't a person worth saving.
An Age of Ultron tie-in has a harsh aversion. Ultron's drone army are just mechanical drones, but they're considered just alive enough to kick the Ebony Blade's curse (which is fuelled by taking life) into overdrive, leaving the Black Knight out of the action.
Invoked in an issue of U.S.Avengers where Colonel Maverick stops fighting Deadpool because he was speaking English. Deadpool appropriately lampshades this.
The slaughter of the Skrulls in Secret Invasion has already been pointed out, but the particular example of Hawkeye and Mockingskrull stands out. This Skrull is a Tomato in the Mirror who is a perfect copy of Mockingbird (Hawkeye's late wife) in every way, including memories and personality, and very honestly believes it when she professes to be her. At first, Hawkeye believes her, too, since she thinks and acts exactly like her. But when he finds out that she is Not Even Human, he kills her in a fit of rage — Even though she had done nothing villainous whatever until then, and did not appear about to. In fact, she died confused and pleading with him, never understanding why her Clint would want to murder her.
When Red Sonja rescues the chef Gribaldi from a cannibal tribe he explains he's been releasing the human captives and cooking foraged lizards and eggs instead. Sonja recognizes them as Lizard Folk offspring immediately before the lizards attack to seek vengeance. She attempts to negotiate with the lizards but concludes they're Always Chaotic Evil. Gribaldi is nonetheless horrified and apologetic.
In Deathstroke (Rebirth) after Slade has a Heel–Face Turn and tries to be a hero, he admits missing being able to grab his sword and kill his opponents and that he considers applying to Justice League to kill some aliens or other monsters during next alien invasion.