An easy way to tell a villainous (or sinister) character apart from the protagonists, other than giving him/her a Red Right Hand, is making their eyes red. If they don't start out evil, there's a chance they become so over time or perform full-blown Face-Heel Turn. It's not the same thing as the "red eye" phenomenon in photos, but if cameras could detect evil, or danger these would be the first people they'd catch.
Even those who aren't truly evil or dangerous are often thought to be so by characters In-Universe.
In the progressive algorithm of Uh-Oh Eyes we have:
Supernatural Gold Eyes < Red Eyes, Take Warning < Hellish Pupils < Glowing Eyes of Doom
Those with glowing red eyes are almost without exception thoroughly evil or, at least, verydangerous. The "almost" section goes to those whose red eyes serve as a marker for their Elemental Powers, typically fire.
A subtrope of Technicolor Eyes. May be part of Evil Albino, or averted/subverted by the Heroic Albino, although most humans with albinism in Real Life have pale gray or blue eyes.
Compare Glamour Failure. Contrast Red Is Heroic.
Not to be confused with the film Red Eye or the liquor. Sometimes a part of Burning with Anger.
It should be noted that trope is NOT just for characters who have eyes of this colour, but when the colour actually indicates the character is evil or dangerous, or presumed to be because of their eye colour. Please do not add characters who just happen to have eyes of this colour without the proper reason behind it.
The giant construction robot that goes berserk has a single glowing red eye that moves back and forth in its head like a 1970's Battlestar Galactica Cylon.
"Hydra". The giant three-headed electrical monster has many red eyes in lines along its head and body.
Black Butler's Sebastian Michaelis has red eyes that are sometimes paired with glowing Hellish Pupils. Despite being a loyal butler to his young master Ciel, he's basically just tenderizing his soul until he's ready to devour it. Sebastian is very much a demon. Literally.
Subverted with the Undertaker. His eyes are green / gold instead of red, but in this context much the same. That is, a very, very bad sign.
Bleach. Hollows are extremely dangerous spirit creatures.
Anime episodes 2 and 3. The Hollow form of Orihime's brother had red eyes.
Anime episode 9. The hollow Grand Fisher had glowing red eyes.
Anime episode 210. After Captain Kensei Muguruma underwent hollowfication, he had glowing red eyes under his mask.
Anime episode 214. The hollow that attacks Don Kanonji and Kisuke Urahara and the giant hollow fortress have red eyes.
Anime episode 220. Captain Komamura's bankai, a giant samurai warrior, has glowing red eyes.
Anime episode 260. A "sword beast" (rogue zanpakuto spirit) has red eyes.
Misa gets these after she receives the Shinigami eyes. The red eyes also show up for other characters who make this "trade", like Kyosuke Higuchi, Teru Mikami, and Soichiro Yagami. In these cases, the red eyes are a stylistic representation of the power for the audience, but it's implied that there's no visual change in-universe. The Live-Action Adaptation instead uses Supernatural Gold Eyes.
Red eyes are also used metaphorically for Light when his Kira persona shines through (i.e. when he's acting particularly diabolical).
An inversion in D.Gray-Man: If Allen's cursed left eye turns black with red concentric rings, you should probably start running. However, it's not a matter of running from him - it's because he just spotted an Akuma and you really don't want to be around when there's bullets flying through the air that can make you dissolve.
All the other Diclonii, too, at least in the anime. Well, definitely Mariko. Nana is more debatable — she's certainly dangerous, being a Diclonius and all, but doesn't seem particularly fond of killing people unless she thinks it's right.
TheEnd from Eureka Seven has red pupils - though its eyes are more disturbing because of the way they roll around in their sockets. But, of course, its eyes turn green after it makes a Heel-Face Turn.
Zeref from Fairy Tail usually has black eyes, but once he's sufficiently pissed off his irises will turn red. Since he's just about the only character in Fairy Tail who can successfully kill people, intentionally of otherwise, this is a sign that you should run.
The demonic nobleman in The Fantastic Adventures Of Unico has reddish eyes in human form. The eyes of any creature that he corrupts or manipulates seem to turn red as well.
Fullmetal Alchemist: Played straight with Scar, then later subverted with other Ishvalans who are, for the most part, victims and refugees of the Ishvalan War. Also subverted when Scar becomes a good guy later on.
Inverted when Al uses a Philosopher's Stone to fight, and his eyes take on a noticeably darker (and incredibly awesome) tone.
Natsume from Gakuen Alice has red eyes in the manga, almost as a warning to the fact that he's a flame-caster (and will most likely try to burn you when he's angry at you). Though, in the anime, his eyes are brown.
Motoko Kusanagi of Ghost in the Shell not only has bright red eyes, but chose the color herself. She's a very scary lady, to the point of remarking casually she's familiar with how to perform dental torture. Later on, it's revealed that she chose the red eyes when she was cyberized at a very early age, and chose them along with her purple hair for no real reason other than the fact that kids like bright colors, then stuck with them because the hair and eye colors had become a part of her identity. But the "dangerous red eyes" trope still very much applies to her present day self.
Shinn Asuka, the (initial) protagonist of Gundam SEED Destiny, has red eyes. He's not evil, per se, but has definite anger issues.
A mecha example in this one, Gundam 00 features this in the Gundams. The primary celestial being units feature green particles, eyes, and clear pieces. The units that are not using the true GN drives feature the same pieces in red. Guess which ones are evil?
However, this gets subverted later on in the movie, which features the Gundam Raphael, which also has red eyes and particles, but is most definitely a good guy! And the same can be said of the Sol Braves team, and in fact all of humanity.
The Gundam franchise is full with these, actually; a number of Zeonic mobile suits are shown as red-eyed Cyber Cyclops, and due to the franchise's love for referencing itself, a large number of suits in non-Universal Century timelines which are inspired by Zeonic ones (mostly the Zaku) have red optics. As a result, more than a few villainous suits have the glowing red 'eye' to warn viewers that they're not good guys. A few Gundam-like units have more human eye arrangements but are also red; this is most noticeable on the Delta Plus from Mobile Suit Gundam Unicorn, which is otherwise mostly shades of blue. This suggests Riddhe's place in the story and his intents, or the type of danger he might pose and that later he'll actually act on his crazed jealousy, enough to try and hunt down Banagher and the Unicorn.
Hajime No Ippo: whenever Bryan Hawk lapses into the deepest pits of his Ax-Crazy, his eyes, excluding the pupils, become completely, bloody red. They even border on glowing. It definitely helps making his batshit insane personality much more terrifying. In the manga (probably due to a lack of colors, duh), the eyes are black.
Hakuōki: glowing red eyes are one of the easiest ways to identify the Rasetsu. Aside from them, there's also Kazama Chikage, who has naturally red eyes and is the most dangerous villain of the series.
In the Hellsing OAV, when Seras Victoria's eyes shift from blue to red (often glowing), expect bodies to hit the floor in numerous pieces. The first Hellsing anime had Victoria's eyes change from blue to red permanently when she first became a vampire.
But the character who really takes the cake with this trope is Alucard himself. Especially when he makes his Nightmare Faces. Oh, and if two red eyes aren't scary enough, how about a thousand of them?
Subverted with Kirara who was first introduced as a snarling beast with glowing red eyes. Then it's revealed she's a friend of Myouga's and was just trying to protect the bodies of the villagers who had cared for her. She becomes a friend and ally after that.
Naraku's eyes are naturally red. As the Big Bad, this is not good from the outset. When they begin to glow, however, true nastiness is about to happen.
Subverted with Kagura. Her eyes are a solid red without pupils. Although introduced as a villain, Character Development makes her an Anti-Villain who tries to help Inuyasha's group, supports Sesshoumaru, and who ends up having an Heroic Sacrifice to save Kohaku's life.
Inuyasha and Sesshoumaru both have gold eyes, but when their sclera turn solid red, very bad things are about to happen to someone.
In the Iron Man anime, Dio's eyes briefly turn red when it goes berserk.
In an odd twist, Guu from Haré+Guu has red eyes in her "normal" form, but considering that she's that world's rough equivalent of Cthulhu, it makes sense.
Subverted in the original Magical Girl Lyrical Nanoha. The red-eyed Fate Testarossa is introduced as an adversary and an apparent villain, but it quickly becomes apparent that she's the enemy more by circumstance than choice, and she does a Heel-Face Turn near the end.
Double subverted in Murder Princess. Alita and the rest of her family have red eyes, which no one comments on and are presumably normal. When Falis and Alita switch bodies, however, red eyes are the only warning you'll get.
The Uchiha clan might as well be built around this trope. Every Uchiha's eyes turn red when using the Sharingan, and Itachi and Obito pretty much always have them on.
Used to some degree whenever Naruto used Kurama's chakra and his eyes turn red (with the added bonus of Hellish Pupils), though he is still aware of whose side he's on. When he reaches four tails and above, his eyes turn into glowing holes, and that's when everyone knows they're screwed.
His eyes turn Orange Red and later pure red with his human pupil when he uses Nine-tailed Chakra Mode and Biju Mode.
Filler character Ranmaru has red eyes when he's using his unnamed eye jutsu. He's technically not evil, just willing to assist Raiga even if it involves helping him kill people, until he sees the error of his ways.
In Nausicaä of the Valley of the Wind, the gargantuan, pillbug-like Ohmus' rows and rows and rows of blue eyes shine bright red when they're enraged. Everything in their path is devastated afterwards, and the spores that collect on their bodies expand the Sea of Corruption a little bit more with each stampede.
Break also has reddish eye(s) and it turns out that, back when he was the illegal contractor Kevin Regnard, he was also known as the Red Eyed Ghost.
Oz gets these when using B-Rabbit's power.
This trope is combined with Apocalypse Maiden for the in-universe concept of the Children of Misfortune. It is believed that children born with red eyes are inherently dangerous, causing tragedy simply by existing. Glen Baskerville was responsible for making them Deader than Dead, since their red eyes are a sign of their ability to influence the core of the Abyss.The recent informations reveals that everything about the Childern of misfortune were lies and that it's the Jurors that they were a threat to not the world because they were created by the core and because of this they were able to bypass their control and harm them which is why they wanted them to be destroyed.Oswald knew of this and used Vincent to scare them.
Also, Mytho, when he's possessed by Raven's blood in the second season...although they're almost more of a pink than a red. Considering the Monster Raven and his servants also have red eyes, it's probably a sign of Raven's blood.
Subverted with Psychic Detective Yakumo. The hero has one red eye, which means he can see the dead. However, it's normally hidden with a contact lens. However, the main villain and Yakumo's blood father has two red eyes.
If Tsukune EVER gains these, run. No if and/or buts about it. It either means he's angry or worse...
Rurouni Kenshin: Makoto Shishio has narrow red eyes. His intro episode is appropriately titled "Devil of Vengeance". Earlier on, though, protagonist Kenshin Himura himself was exhibiting the trope as Hitokiri Battousai until his battle with Saitou Hajime gave him gold eyes, probably to match the prescribed manga coloring.
Sailor Moon: Played straight by Black Lady and Galaxia before Sailor Moon 'befriended' her. Beryl's eyes were also red.
The Seven Shadows in the series have them to the extremes. The ENTIRE eye is red. Sclera, iris, AND pupil.
Fiore gets these when merges with the Kisenian flower. After his defeat, he gets these when he tries to pull an If I Can't Have You situation on the Senshi by crashing the asteroid they're all on into the Earth. Love Makes You Evil indeed.
Saint Seiya. Gemini Saga's eyes turn from a pleasant blue/green to bright red and glowing whenever his evil side takes over. When this happens, it's a good idea to run away very, very fast...unless you have a death wish, are The Hero, or both. His hair changes colour too.
Inverted, discussed, and ultimately deconstructed for Gojyo in Saiyuki: his red eyes are considered a bad omen because they mark him as a half-breed when paired with his identically-red hair, but this really only turns out to be unlucky for Gojyo himself, as he keeps getting treated as an outcast because of them.
Demon Eyes Kyo from Samurai Deeper Kyo is the living incarnation of this trope. Even more impressively, he manages to do this and Glowing Eyes of Doom simultaneously. In an interesting subversion, the nature of Kyo's eyes and his connection to the enigmatic Mibu clan become central points to the plot.
Ruby-Eye Shabranigdo, the supreme demon lord and Sealed Evil in a Can from the first season of Slayers. Lina herself has them too, but while she's not a vilain, her record with property destruction certainly makes her dangerous.
Shinryaku! Ika Musume. Ika Musume does this in the anime when she gets angry and unleashes her full power. Watch a brief example here.
In Tekkaman Blade, all of the enemy tekkamen had red eyes and red crystals. When Blade exceeds his time limit and goes crazy, his eyes turn red as well.
Present in the sequel too. Dead End had red eyes before his Heel-Face Turn.
Silky from Tenshi Ni Narumon has red eyes. In the first season, it is a subversion, but in the second one, it is played straight because she is a villain - of a raging psycho bitch variety. At the end, it is subverted yet again, because it turned out she really is a Woobie.
In Umi Monogatari, this is averted with Kanon and the shrine maiden, but played straight with a corrupted Urin.
Mutsumi from Utawarerumono has red eyes (as does Kamyu when she goes into vampire mode).
In Vampire Knight, the vampires' eyes turn red when they are suffering from blood lust. And Kaname's eyes are just red in general, and he's not exactly the most benign person...
Dilandau in Vision of Escaflowne has garnet-colored eyes and platinum hair, and he's totally crazy and bloodthirsty. Turns out, his true form is a blond haired and blue eyed girl, so the hair and eyes are a side effect of the experiments performed on him.
Fuuma's◊ eyes in the X1999 anime series are dark red, in contrast to his manga counterpart, whose eyes can best be described as gold-hazel (giving him something of a cat-like appearance). The change in eye color probably has to do with the taken-for-granted aspect of his true nature in the anime series, while the manga builds up much more subtly towards the Wham Episode and his Face-Heel Turn.
In the Toei-produced "Season Zero" of Yu-Gi-Oh!, Yami's eyes were red. And bad guys really should take warning, because not only was Yami completely psychotic originally, but anyone who messed with his friends either ended up dead or even crazier than he was.
Kurogane from Tsubasa Reservoir Chronicle has red eyes and appears to be a rather dark at the beginning, given his apparent bloodlust and over-enthusiasm for killing people who go after his Princess. Subverted though when it's established that he is indeed a pretty good guy and probably the only one who is completely there amongst the main characters.
Briefly in episode 22 of Attack on Titan Levi's eyes become red when attacking the Female Titan for killing his squad. Take warning, indeed.
One Piece: Sort of, but if you ever see Luffy looking at you with his pupils not fully black, RUN.
YuYu Hakusho: Some of the demons. Though many demons appear to have this trait not as a default eye color, but whenever it appears that they've tapped into their demonic nature overlapping with Glowing Eyes of Doom. Reminding you of their nature.
Mega Man himself as he was infected by Wily's malware.
One of the first signs that something is up in Flashpoint and its cartoon adaptation is that Batman's got red eyes/visor lenses instead of white... because Bruce died that night in Crime Alley, the Batman in Flashpoint is his father Thomas.
Lampshaded in the second story-arc of My Little Pony: Friendship Is Magic (IDW); when Applejack spots a group of cute Nightmare creatures, the nearest of which have sparkly red eyes, she asks Fluttershy for her opinion on their significance:
The New52 version of Superboy, when using his telekinetic abilities.
Superman, big time. Granted, his eyes nearly always glow red just before he uses his heat vision in any context, but Superman with BIG glowy red eyes and an angry expression fits this trope and means that someone's about to get seriously hurt. Notable examples include For the Man Who Has Everything (to Mongul: "Burn"), "JLA/Avengers" (in his fight against Thor), and "Kingdom Come", where he had to be talked down from murdering the entire UN Assembly in the final chapter. But perhaps his best display is when Darkseid apparently killed Supergirl in the Superman/Batman arc, and Superman, after shedding some tears for his cousin, donned his red eyes of doom and gave Darkseid the beatdown of a lifetime.
Subverted in Lex Luthor: Man of Steel. Superman is always presented with red eyes, but it's due to Luthor's Unreliable Narrator perspective. And the fact that whenever Superman and Lex Luthor are face-to-face with each other, the reason behind Superman's glowing, angry red eyes is usually because Luthor's just done something utterly monstrous and gotten away with it.
According to the Marvel handbook, this should be the case for Werewolf by Night: his eyes are yellow when he is transformed and fully in control of himself; when he transforms involuntarily and becomes a dangerous, wild animal, they are red. Unfortunately the colorists don't quite adhere to this and the two often get switched around.
If X-23's eyes turn red, run. Run away very fast. It means she's under the effects of the trigger scent and you're about to die a horrible, meat confetti death.
In A Pikachu in Love, this is the first thing Pikachu and Pichi see after accidentally crashing into an Ursaring's den...
Dungeon Keeper Ami features this prominantly. Keepers all have red, glowing eyes; these glow brighter when feeling strong emotions, such as anger. This actually becomes a plot point once Ami makes contact with her own world, and is the basis of Tiger's rather annoying prank.
Averted and played straight in My Immortal. Vampires Ebony and Draco either have red eyes or wear red contacts, and are (supposedly) good guys. Played straight in that Vloxemort, the Big Bad of the story, is frequently described as having red eyes.
Mrs. Jumbo from Dumbo normally has blue eyes like her son, but they turn red during the scene where she goes crazy as a result of several children making fun of Dumbo, resulting in her being tied up by several circus workers and subsequently being dragged away to be locked up in solitary confinement as punishment for being a "mad elephant".
In The Black Cauldron, the Horned King's eyes go angrily crimson after his army of undead warriors has been halted.
Aladdin. While Aladdin and the Genie are in the cave, Aladdin taunts the Genie about the limitations on his wish granting abilities to trick him into getting everyone out of the cave without using up a wish. When the Genie gets angry and starts giving a speech, his eyes turn red momentarily.
In Treasure Planet, Silver's cybernetic eye is usually a yellowy-amber-orange-ish colour, but it changes to red when he gets angry/displeased.
The PNK sorority girls in Monsters University. They're usually bubbly valley girl types, but when they get down to scaring, yikes.
Spoofed in Monsters vs. Aliens. After being imprisoned by the military merely for being a 49 foot 11 inch monster woman, Susan starts banging on the walls of her prison. The other monsters warn her not to do that, whereupon a huge door opens to reveal a dark interior with two glowing red eyes within...which merely turn out to be the lights on General Monger's Jet Pack.
Played straight with the alien probe. When its eye turns red, there's little hope for you if you're in its way.
The Fabrication Machine and all its robot minions in 9.
In Superman vs. the Eliteonce Superman decides to fake crossing the Moral Event Horizon, not only do his eyes glow red. The blood vessels in his left eye were damaged earlier in the fight, giving that eye a reddish tint, which only adds to how shit-scary he is at the time.
In The Thief and the Cobbler, all of the One-Eyes (an evil race that plans to destroy an idyllic Golden Land) have a single, big red eye.
A rare yellow colored variation of this trope happens at the end of Angel Heart when Louis Cypher is telling Harry Angel that he was the missing cannibal serial killer Johnny Favorite and that Johnny's (And therefore, Harry's) soul is his, for a brief couple seconds, his eyes turn yellow.
The Fog (1980). Blake, the leader of the zombie ghosts, has glowing red eyes.
Vampires in Fright Night (1985). Jerry Dandridge and Amy Peterson in their humanoid forms, Jerry Dandridge in bat form, and "Evil" Ed Thompson in wolf form all have red eyes. Hammer Horror vampires often have red — or rather, very bloodshot — eyes.
In Godzilla Tokyo SOS, the title character tries to charbroil Mothra's newly borne twin larvae with his atomic breath. Mothra jumps in front of her children to save them and is promptly set alight for her troubles. She explodes and, at that point, her children's eyes turn from a benevolent blue to a hellish red and they proceed to team-up with Mechagodzilla to beat the hell out of Godzilla.
Space-Godzilla has glowing, red, eyes with cat-like slits as opposed to Godzilla's fiery, human-like ones.
The Mutos from Godzilla (2014) have red glowing slits for eyes, which lack any kind of pupils. They may be heat sensors or compound eyes.
Mogwai in the Gremlins series when they've eaten after midnight and become evil.
It's not the eyes, but the robots in I, Robot turn on a red chest light when being controlled by the villain. Justified: it's explained that the red light indicates when a robot is downloading a firmware update from US Robotics. VIKI uses this connection to upload her (twisted) logic and understanding of the Three Laws into the consumer-model NS-5's.
In Iron Man 2 The Mk II eyes light up blue like all Iron Man suits—as War Machine, they are red.
In Judge Dredd, the deadly ABC robot controlled by Rico has glowing red eyes.
Oblivion (2013) : You didn't expect the HAL 9000-eyed drones to be up to any good, did you?
One Hour Photo has Robin Williams in a calm dream sequence, but when he opens his eyes, they are blood red and some blood trickles out of one. He then puts his hands on his eyes, screaming out "AAAAHHHHHHH!" while blood flies all over the place from his eyes.
Popeye and a very late Olive Oyl in The Movie carrying the newly discovered Swee'Pea at the door of Olive's house after being late for a date with Bluto=big trouble. Bluto, seething with anger, sees Popeye, Olive, and the baby through red eyes, pummels Popeye into the ocean below, and, in the next scene, persuades the taxman to tax the prestigious Oyl family into bankruptcy as revenge.
Subverted in Spaceballs. When the baby xenomorph bursts out of John Hurt's chest, it has red eyes, but it immediately goes into its "Michigan J. Frog" number and dances away.
Star Wars: the "Sith eyes" seen on Darth Sidious, and later Darths Maul and Vader in the prequels (technically gold rimmed and flecked with red, but it still counts) Maul is shown to have them all the time, but he's not human and his eye color is never seen while he's doing anything but hunting the heroes and fighting. Vader is only seen with them when he's feeling particularly "eeevil." Sidious' eye color, on the other hand, switches between Sith gold and their original blue depending on whether he is being his evil self or masquerading as a benevolent politician. Ultimately, his eyes become gold-red permanently after his gruelling duel with Mace Windu.
This trope is subverted with Darth Tyranus, who is never seen with Sith eyes.
The eyes of Cad Bane from "Clone Wars", the Nemoidians and Grand Admiral Thrawn may fit this trope better on a superficial level (being pure red), but this is only because that's the natural eye color of their respective races - not Sith sorcery.
In the theatrical version of A New Hope, Vader's eye lenses had a dull reddish tint to them due to the materials used. It probably wasn't intentional, due to the fact that later edits removed the reddishness in order to hide David Prowse's eyes.
In Teen Wolf (1985), Scott's eyes flash red and his voice drops to an evil growl to intimidate the hard-ass old man into selling him a keg of beer.
Most of the Terminators from, well, the Terminator franchise have glowing red eyes, which become even more obvious when only their endoskeletons are showing.
It is probably not coincidental that Cameron has blue eyes under the meat...although the third movie features another quite malevolent Terminator similarly equipped.
Actually, Cameron's endoskeleton has red eyes.◊ The blue light we saw in some episodes was explained by Josh Friedman: "It's red because at the core, she's a Terminator. Plain and simple. The lens itself is red. There'd never be a reason to change the lens color. I made [the flash] blue in the pilot to indicate she was different but that wasn't to suggest the hardware was blue. No reason for it." Some theorized that the blue eyes were some sort of Resistance Security Encryption Key, considering its use in unlocking access to both the TDE station in the vault (for the Connors) and the TDE complex in the future (for Derek).
The Hood, the villain of the live action Thunderbirds film, has these when he uses his psychic powers. His heroic niece, Tintin, has Supernatural Gold Eyes in contrast, when she uses her powers.
The Infected in 28 Days Later have red eyes. They're also very aggressive.
The HAL 9000 computer in 2001: A Space Odyssey uses cameras with glowing red lenses to watch the crew of the Discovery.
In Who Framed Roger Rabbit, Eddie Valiant remembers that the toon who killed his brother had "burning red eyes". That toon is later revealed to be Judge Doom.
The Wizard of Oz (1939). In the Haunted Forest, Dorothy and her companions see two evil owls and two vultures. All of these birds have glowing red eyes.
The direwolves have red eyes. The ones the Stark children have are not evil, but dangerous indeed.
Only Ghost has red eyes, and that's because he's an albino. The other Stark direwolves have yellow, gold, or green eyes.
One of the villains from Madeline L'engle's A Wrinkle in Time is called simply...The Man With Red Eyes.
Older Than Print: the monster Grendel from the Old English poem Beowulf is described as having glowing red eyes: "on fagne flor feond treddode,/eode yrremod; him of eagum stod/ligge gelicost leoht unfćger" ("Across the decorated floor the fiend trod, went enraged in mind; there stood in his eyes a foul light most like to flame").
Red eyes seem to imply evil thoughts/feelings. Young Tom Riddle's eyes gleam red briefly at a moment when he is extremely greedy, and later, when he has started turning into Voldemort, his eyes become bloody-looking - they ultimately end up staying pure red after he splits his soul seven times. Also, when Ron is being psychologically tortured by Riddle!Harry-and-Hermione from the horcrux, his eyes flash red for just a second...before he ignores them and shuts them up for good.
Notice the formidable chiasmus at the end of Goblet of Fire. Voldemort has red eyes and uses a green Death Ray. Harry Potter has good green eyes, and uses the good red ray (remember the color-coding of laser beams in the original Star Wars trilogy).
The villain Sauron, while not red-eyed himself, uses a red eye on black as his symbol.
In The Two Towers, the Műmak (giant elephant) who tramples the Ithilien Rangers and men of Harad is said to have raging small red eyes.
In The Return of the King chapter "The Scouring of the Shire" it says that Saruman's evil agent Wormtongue has red eyes.
In the original picture book of Mars Needs Moms, Milo's mom (and I think Milo himself) have bright orange-red eyes. It doesn't mean anything but it does make The Reveal of Mom's face a little unnerving.
Michael Stackpole's Wolf & Raven short story "Fair Game". The cyberdog that Selene Reece sends after Wolfgang Kies has eyes that look like red starbursts.
Voidbringers in The Stormlight Archive are alleged to have red eyes, and we see at least one form of Voidbringer, a thunderclast, with glowing red eyes. Furthermore, in Words Of Radiance, the Parshendi's eyes turn red when they transform into Voidbringers.
The plus-sized Tyrannosaurus rex in Dinoverse, who Mike names "Moriarty", is first seen with eyes that seem to glow red, though Mike thinks it's a trick of the sunset or a reflection of the blood at the dinosaur's feet.
Live Action TV
Angel: Lorne's got red eyes because he's a demon, but he's a gentle soul who's pretty sweet and fun company.
The second Vorlon Ambassador in Babylon 5 has a red light in the "head" of his encounter suit, compared to the original Kosh's green. He's a much, much nastier person.
Battlestar Galactica: Cylons. The single red flashing eye makes up a great deal of the Centurions' and Raiders' intimidation factor. Strangely, the only time we see a humanoid Cylon with red eyes, it's Sam Anders, one of the good ones.
After Tara is killed, Willow looks up, and her eyes are glowing red. It's even lampshaded/foreshadowed by the episode's title, "Seeing Red".
Red eyes are possessed by the First Evil's briefly-seen true form.
Charmed: Used on Inspector Rodriguez to confirm him as an upper-level demon.
When the placid "servant race" of Ood in Doctor Who get red eyes, it's a sign that they've either been posessed by a Satan-like being from before the universe began or simply overwhelmed by anger at the way they've been treated. Either way, it's bad news.
Meanwhile, the only human that "Satan" managed to possess also gained blood-red eyes while the Beast was in control.
Far earlier, those possessed by the similarly diabolical Fenric gained blood-red eyes, but only after they had faded from their initial shade of dark green.
In the serial Robots of Death, the eponymous robots' eyes are filled with red static when they prepare to kill.
Similarly, the Siren from The Curse of the Black Spot turned not only her eyes, but the rest of her projected body from blue to red when anyone got between her and her victims.
On Farscape, if a Delvian's eyes turn red, be moving in the other direction. It means they're going insane.
On Grimm, we've seen Monroe's eyes turn red when he's upset, but not ready to go full Game Face in that situation.
It's worth mentioning that much of the time the person's eyes are normal color. It's only when they are feeling particularly evil that their eyes turn red, and even then it's merely for a second.
Darien Fawkes of The Invisible Man isn't evil, but he's prone to states of psychosis induced by the invisibility-causing gland in his brain; when he starts to flip out, his eyes go red. But when he gets to the point of no return, his eyes turn silver.
Mituaki Gamou, the Big Bad of Kamen Rider Fourze, has eyes that often glow red and was initially known as "The Red-Eyed Man" in promotional materials.
Inverted with Kamen Rider Kuuga. Red Eyes Ultimate Kuuga is a GOOD thing as it shows Godai's good nature is shining through.
In Merlin, Gaius realises that two visiting nobles are actually evil enchanters as he sees one's eyes flash red in a fit of anger. He really does take warning at the red eyes.
In Star Trek: Enterprise, the Na'kuhl (or, as they are better known, the "Red Eye Aliens" or "Evil Alien Nazis") all have red eyes, clinching them as candidates for the most evil-looking aliens in the history of fiction.
Jake Sisko and, later, Dukat temporarily develop glowing red eyes while possessed by the Pagh Wraiths in Star Trek: Deep Space Nine.
In the Star Trek: The Next Generation episode called "Imaginary Friends" the titular girl, Isabella, gets red glowing eyes when she is upset when Clara leaves her at Troi's suggestion.
Supernatural: Crossroads Demons. Other demons have similarly evil shades of white, yellow, or black.
Teen Wolf, Alpha werewolves' eyes turn red when they feel angry or predatory, even in human form. Once they shift, their eyes may glow outright. Also, Scott's yellow/gold eyes whenever he transforms.
In the original V series, the Visitors' true reptilian forms had red eyes.
The X-Files, episode "Detour": MOTW creatures Moth Men had glowing red eyes. Very scary.
The Sweet's "Ballroom Blitz" mentions this about the man in back. "As a matter of fact, his eyes are as red as a sun." He's responsible for it turning into the titular ballroom blitz.
David Bowie's "Cat People (Putting Out Fire)", written for the 1982 film, has a lovelorn man singing to an object of desire, warning her of the intensity of his need. The first verse establishes that his eyes are usually green, but they become red in the second verse:
See these eyes so red Red like jungle burning bright Those who feel me near Pull the blinds and change their minds
Lilo I, and probably every other vampire, in Bloodline gets this once they drink someone's blood. This gives the showing that she's about to kick ass.
Gorillaz bassist and Magnificent Bastard Murdoc Niccals used to have one red eye and one black eye. The red eye appears to have turned black in recent pics, which may mean anything or nothing story-wise.
The eyes of cemetery statue "Black Aggie" are said to glow bright red in the middle of the night, either blinding or killing anyone who looks into them (source). One Urban Legend/Creepy Pasta talks about a man who stays at a hotel but is told to stay away from a certain room. Out of curiosity, he peeps through a keyhole and sees an apparently normal room, with a woman sitting against a wall. When she moves, he backs away and returns to his room. Later, he tries looking in again, but he can only see something red through the keyhole. Not able to stand the questions any longer, he asks the hotel clerk what's wrong with the room. She tells him that, years ago, a couple died inside, and they tended not to mention it to avoid upsetting guests. She also mentions that the couple had unusually red eyes.
Garfield's art style usually doesn't allow for eye colors at all, but the infamous "abandoned house" sequence (beginning here) has a close-up of one of Garfield's eyes in the final strip, red like blood, with the pupil contorted into a spiral as he refuses to accept what he sees.
Another very early strip around Halloween, Garfield explains that around that time cats become bewitched. His eyes began to turn red among other things... that is until he saw himself in the mirror, screamed turned white and passed out.
Al Capp's Lil Abner strip had the Nogoodniks, which were the evil version of the Shmoos. They were a sickly shade of green and had "li'l red eyes". They devoured "good" Shmoos, were the sworn enemies of "hoomanity," and wreaked havoc on Dogpatch.
SCP Foundation, SCP-1111 ("The White Dog"). The title ghostly animal has red eyes that glow more brightly the closer it is to its point of origin. It savagely attacks all human being that approach it.
The antagonist in Centaur has small red eyes, as if his bestial face, inhuman form, and dominatrix-inspired design didn't raise enough warning flags.
Similarly, the monstrous Varkon has round red eyes, which combine with his small fangs and rounded, brutish face to complete his Monstrous Humanoid appearance.
All the Worlds' Monsters supplement by Chaosium.
The Chaotic Evil Coachman of Death and his horse have glowing red eyes.
The Lesser Demon has glowing red eyes.
The Night Hound, which hunts down and kills all Lawful beings, has red eyes.
The Evil undead Lemure monster has glowing red eyes.
The powerful and dangerous Three Horsemen of Doom all have flame red eyes.
Arduin Dungeon #2 The Howling Tower has a deadly steel tiger monster with red eyes.
Ars Magica. Brude Deathless, the oldest and most powerful gruagachan in all Scotland, has a terrible temper and loves to put curses on anyone who annoys him. His eyes turn red when he gets angry.
Atlantis: The Lost World generic RPG setting. Wraiths (which are based on J. R. R. Tolkien's Ringwraiths/Nazgűl) have two glowing red eyes that are hidden in shadow. Bat Horins are lesser devils whose eyes radiate an eerie crimson luminescence.
Blood Of Heroes. Both the supervillain Lord Bane and his horse Hell's Mount have glowing red eyes.
Deities: Atlach-Nacha (Great Old One), the Haunter of the Dark (avatar of Nyarlathotep), Ithaqua.
Monsters: Black Unicorn of Averoigne (Worlds of Cthulhu #3, "Dark Ages Averoigne"), Bunyip (Terror Australis), ghouls (glowing red, Shadows of Yog-Sothoth adventure "The Warren"), Necromantic Skeleton (The Asylum and Other Tales - "Black Devil Mountain"), Proto-Shoggoth (The Asylum and Other Tales, "The Asylum"), rats (Masks of Nyarlathotep - Kenya section), Spectral Hunters, The Thing from Between the Planes (The Fungi from Yuggoth), Undead Sorceress (Dreamlands adventure "Pickman's Student"), Werewolves (Worlds of Cthulhu magazine #3, adventure "Malevolence").
Supplement Dark Designs, adventure "The Menace from Sumatra". Vibur (AKA The Thing from Beyond) has three triple lobed burning red eyes.
Carcosa: Weird Science-Fantasy Horror Setting. In hex 0814 is a Spawn of Shub-Niggurath of Chaotic alignment (worships and serves the extremely evil deities of the Cthulhu Mythos). It has a single red eye.
In the supplement Enemies II the supervillain The Monster has glowing red eyes.
In the adventure Deathstroke, the supervillain Doctor Death wears an armor suit with a deep red light coming from his eye sockets.
Mayfair's Chill. Evil game monsters that have red eyes: Barghest, Ghoul, Hook Thaggis, Spectral Hound, Werewolf (wolf form).
Chivalry & Sorcery 3rd Edition adventure Stormwatch. Kobolds living in a copper mine have been kidnapping, killing and eating human miners and other villagers. They have ruby red demon eyes, red as sparks and hellfire.
Dangerous Journeys/Mythus, Journeys magazine issue #2. In the adventure "It's Never a Good Day to Die", the evil necromancer Horace Quintell has eyes that burn with an intense redness.
Most versions of drow or dark elves have red eyes. They are Always Chaotic Evil and the exceptions inevitably have other eye colors such as violet or yellow. They all have red eyes in deep darkness, however. It's part of their darkvision.
Interestingly, the original incarnation of drow as presented in the modules in which they made their first appearance were described as having amber or yellow eyes. It was only in later developments that they were described as red-eyed, possible a case of Fanon due to the misconception that the heat radiance of thermal imaging organs (or ultraviolet imaging organs) appear red to normal vision.
Creature Collection: Ratmen/Slitheren (all types have red, beady eyes)
Kara Tur (Oriental Adventures) modules
OA5 Mad Monkey vs. Dragon Claw. The Chaotic Evil vampiric ningyo (mermaids), two Dragon Claw warriors the PCs kill, and the giant Dragon Claw statue after it's animated.
OA6 Ronin Challenge. The powerful Lawful Evil Earth Spider and its minions (goblin spiders, giant spiders, and bakemono) all have red eyes.
OA7 Test of the Samurai. Bokika Hokio, the Lawful Evil 16th level samurai daimyo of Fochu province, has fiery red eyes and an explosive temper. The krakentua monster is Chaotic Evil, 80-100 feet tall, looks a lot like Cthulhu and has red eyes.
Module X2 Castle Amber. In the Indoor Forest, the master of the Wild Hunt (Andrew David Amber) has glowing red eyes.
Module O2 Blade of Vengeance. Gorkalk, the great grandfather of the Chaotic Evil red dragon Khordarg. It may be that all red dragons in Basic D&D had them.
Module B8 Journey to the Rock. The ghostly horsemen of the ruined city of Tuma have eyes that are dull red, like a dying fire.
1st Edition module I12 Egg of the Phoenix. The Lawful Evil wererats in the "Crypt of Empyrea" adventure have red eyes. This was not stated in the 1st Edition Monster Manual entry on wererats.
The PCs are sent back in time to when Strahd von Zarovich was changed into a vampire. After the conversion the Chaotic Evil Strahd is stated to have glowing red eyes, a dead giveaway to the fact that he's now a supernatural creature.
Azalin's phylactery takes the form of a huge 200 pound gold-plated dragon skull. Its eyes are rubies and an ever-burning flame is in its mouth, giving the eyes a dull red glow similar to that of the Lawful Evil lich it is magically linked to.
Issue #25 article "Varieties of Vampires". The Ch’ing-Shih vampire from China has red, staring eyes.
Issue #51 story "A Part of the Game''. A monster that used its Your Heart's Desire ability to lure 12 men to their deaths has "two red eyes burning like coals in a brazier in the Hall of Pain."
Issue #93 article "Nine Hells Revisited". The powerful Lawful Evil greater devil Malarea has fiery red eyes.
Issue #142 "Ecology of the Kech". The Neutral Evil keches have blood-red, pupil-less eyes. They are extremely evil and rapacious, considering all other creatures (including humans) to be food. They love to set up traps to catch victims.
Al-Qadim boxed set A Dozen and One Adventures. The black idol in Al-Anwhar has two ruby eyes that glint malevolently. If the PCs touch the eyes, they can be Mind Controlled into sacrificing themselves to the idol.
World of Tabletop Game/Greyhawk Fantasy Game Setting boxed set (1983) "Glossography" booklet, adventure "The Lost Passage of the Suloise". The title passage is guarded by a huge monster with glowing red eyes (which are actually gems) left by the Lawful Evil Suloise. If it catches a victim it will crush them flat.
Mayfair Games' Role Aids supplement Dark Folk
The Chaotic Evil gnoll deity Xyrmgnollven has blood red eyes.
The campaign setting: Wraiths, Wild Hunt hounds, Zephyr Hound - Fire (glowing red), Demons of the Sixth Pale (glowing red), the Dragonlord Oran Jatar, Black Unicorns (glowing red), the Heralds of Night (glowing red) and the Dark Gods Orgiana (glowing red), Z'taar (luminous red), and Scalar (luminous red).
Quellbourne: Land of the Silver Mist. Trolls have deep-set red eyes.
Demons of the Burning Night. The following monsters/opponents have red eyes: Flete Demons, the greater evoker demon Teroglustrod The Red Gate and the half-demon General Mortilas.
Journey to the Magic Isle. The lich Balizaar, the deadly and evil enemy of the University of Magical Arts, has glowing red eyes.
Jaiman: Land of Twilight. Black trolls (AKA War trolls) are dangerous, fearsome and cruel man-eaters that have evil red eyes.
Space Gamer Fantasy Gamer magazine #7:
Article "Villains Finish First!". The super villain Gloom has blood-red eyes.
Article "Barony - A Tale for a Bored Court". In Barony, dwarves are base, evil creatures which have strange red eyes.
The Big Bad Markos Keras has dark red eyes, unusual for an Orion. His servants think they give him supernatural powers. He has a fierce temper, is rash and impetuous, and is likely to try to have the PCs killed.
Alien animals called Slethi have teeth that inject a venom that causes intense, immobilizing pain and ultimately leads to death. They also have bright red eyes.
On Easter Island a villain has 4 statues with eyes that "smolder evilly red''.
The Daven and Abomination demons have red eyes.
The entire ork race in Warhammer 40,000 has evil red eyes. The warning part however usually comes from a mountain of weapon wielding green muscle charging you. Besides, the eyes are quite sunken and difficult to see unless you're up close and suicidal.
Or unless it's dark. The 3rd edition Ork Codex featured cover art of a greenskin mob charging a trench during a thunderstorm. The Orks in the foreground were well-lit, while all the ones beyond were just hulking shadows with tiny glimmers of red glaring out at you.
Witch Hunter: The Invisible World.
The monstrous black hounds that roam the English countryside and attack travelers have glowing red eyes.
The vampires known as Apotamkin have blood red eyes.
It Came from the Late, Late Show II adventure "Bjorn on the Bayou, or Escape from Alkatrazz XII". Alkatrazz XII is a prison planet. Warden Skrank is corrupt, sadistic, and a cyborg with a blinking red robotic right eye.
Zigzagged in BIONICLE. As a general rule, the color of the eyes can be used as an indication for "good or evil", but you have to ask yourself how the eyes fit with the armor.
In the movie Bionicle: Web of Shadows, Toa Vakama inverts this: he has red eyes until he goes over to The Dark Side, at which point, his eyes turn green. His toy, on the other hand, always had green eyes.
The evil Roodaka from the same movie had blue eyes as a standard, but when she got really angry, they turned bright orange/red.
And naturally, the original Big Bad, Makuta Teridax, also had his trademark red eyes (which he liked to project into shadowy corners), as did many of his brethren, as well as some other villains in the toyline (Krekka, Sidorak, Piraka, Keelerak spiders, etc.). Some, however, eventually substituted orange for red, when the eye colors became Color-Coded for Your Convenience. Generally speaking, though, the eye colors of the Bionicle characters were all over the place, especially in the earlier years.
In AdventureQuest Worlds, Chaos Dragonlord Vath's eyes are normally purple, but when his Berserk Button is pressed, causing him to lose his cool after he finds out that the hero freed the dwarves and they are winning against his army, guess what color they turn...
In Anarchy Reigns, Jack's eyes will turn red whenever he gets really angry (and he's already pretty bitter most of the time).
Jake's eyes change to red when he is being controlled by Specter in Ape Escape. Also, Specter's eyes change when he dons the Peak Point Helmet and uses his newly gained augmented intelligence to try and take over the world by changing history.
Lieselotte Achenbach, the Enfant Terrible from Arcana Heart, has red eyes and is nicknamed "The Crimson-Eyed Criminal", as she's an emotionally scarred assassin. She also has white hair. She'd fall under Evil Albino if she were just a bit more pale.
Clarice di Lanza, a nun who is also a demon, usually falls under the Eyes Always Shut trope and is, for the most part, good natured...unless you do something to seriously anger her, at which point her eyes open and this trope is in full effect.
Asura from Asura's Wrath has natural brown eyes. But when he crawls out of Naraka for the first time, his eyes become glowing white orbs with red-white pupils, and he's pissed. At the very end of the final DLC chapter, when Chakravartin is slain and his wrath is finally gone, Asura's eyes revert to their original brown color before he fades away.
Castlevania series usually depicts Dracula as having Red Eyes, and in Portrait of Ruin — the twins Loretta and Stella both have one red eye, and Brauner also has red eyes. The twins' heterochromatic eyes is to show that they are under Brauner's sway but they are still able to be turned back to humans, losing the red eye when they do. However, other vampires such as Elizabeth Bartelby, Carmilla, etc. aren't shown with red eyes, so it's not a guaranteed Vampiric trait.
The Praetorian Police in City of Heroes: Going Rogue occasionally say this, "Its eyes glowed red. That's all I need to take the safety off."
The Black Phantoms of Demon's Souls have red face orifices, which should tell you that they are much, much more powerful than your enemies up to that point and you should consider going back the way you came.
There's also a non-Black Phantom mook in the first non-tutorial level called the Red Eye Knight. Players that don't know what they're getting into often die within seconds of provoking him. Boss in Mook Clothing indeed.
If the eyes of the Dragon God boss turn red, start running and don't stop until you're behind a pillar or other large visual obstruction, unless you like the idea of a fist larger than you are smashing you through a granite wall.
Dante from Devil May Cry, before he goes to town on Mundus. Twice for Nero in DMC4, the first time when he gains his Devil Trigger, and the second when Angelo Agnus escapes with Kyrie and leaves Nero to vent his rage out on some Mooks.
Shadowmere in The Elder Scrolls IV: Oblivion and The Elder Scrolls V: Skyrim. The most badass Cool Horse in the game. Astrid, who gives you Shadowmere, never really tells you what the horse actually is, but Shadowmere is tougher and heals faster than all other horses, and is also extremely aggressive, and will happily take on dragons.
In The Elder Scrolls: Arena this is definitely part of Jagar Tharn's Evil Sorcerer-style, though in-universe the red eyes only marks him as suspect to racists (it marks him as having Dunmer somewhere recent in his ancestry).
The Dunmer themselves zigzag this. Individual dark elves can be as good or as evil as anyone else, but according to legend their coloring, including the red eyes, was a punishment by the Daedric Prince Azura for the Tribunal's betrayal of Nerevar.
In Fable, you can get glowing red eyes if you're evil enough.
Not always red, but Combine soldiers in Half-Life 2 tend to have glowing eyes.
To specify, the OverwatchElite soldiers have a large red eye, and Overwatch soldiers with shotguns sort of apply, with reddish-orange eyes.
They definitely apply; the shotgun wielding Overwatch soldiers are (some of) the most lethal enemies in the game if you stumble across them in close range, so you really should take warning when you see their red eyes bearing down on you.
Halo: 343 Guilty Spark's eye turns red when he becomes Rampant. His Eye Beams turn red as well.
When the Didact undergoes procedures to acquire Flood immunity, his eyes turn from grey to a hellish red-orange. He begins acting a little unhinged (possibly as a side affect of the failed procedures), culminating in attempting to harvest humanity into his personal Flood-fighting robotic army.
Vaati also has red eyes, and he's the Big Bad for three games.
Dark Link often has (glowy) red eyes and is always an antagonist.
Played with in Love And War. Lavender Regale is a bright, cheerful Girl Next Door, but she has also has a violent temper. When she's about to lose it, her eyes turn red. Ryan, the protagonist, lampshades the usual usage of this trope by jokingly referring to her as "the she-devil, Lavie" whom only "the Holy Paladin, Ryan Eramond" can defeat.
In ''Mario & Luigi: Dream Team, both the gardening robot Grobot and the later defence system boss Pi'illodium having ominous glowing red eyes. The former to indicate it's broken and gone insane, the latter just to show it's evil and currently at full power (its eyes temporarily turn blue whenever its wings are destroyed and its defences weakened).
Mass Effect 2 has Shepard, the protagonist, who - if you play him/her as a violent renegade - gets eyes that glow red. This is due to an aftereffect of the Lazarus project: at the start of the game, you are recently brought Back from the Dead and have scars on your face and cybernetic ocular implants. If you play as a Paragon, they heal over time; whereas if you play Renegade, the red glow in Shepard's eyes becomes more pronounced, along with the scarring. Regardless of the alignment, however, you have an option to purchase an expensive upgrade to you medical bay that permanently heals the scars, after which your Paragon/Renegade actions will stop affecting your face.
Forte from Mega Man (Bass in the English version) has these.
Subverted in Neverwinter Nights 2 with Neeshka. Her red eyes are courtesy of having a pit fiend for a grandfather, and tieflings such as her have a tendency towards evil because of their fiendish heritage. However, Neeshka is quite friendly and is at worst a Kleptomaniac Heroine, and can only turn against you because of a Geas placed on her by The Dragon when he captures her in the endgame. She can even break free of it if your Relationship Values are high enough.
Ghetsis, a member of Team Plasma from Pokémon Black and White, takes this trope to new extremes - he has one natural red eye, a glowing red monocle, and a red eye printed on each sleeve of his robe. It becomes fitting as more of his actions and intentions are revealed.
Gyarados. Huge, vicious, and very ill tempered, it has destroyed entire cities.
Quilava and Typhlosion. If Typhlosion's rage peaks, everything it touches bursts into flames
Sneasel is said to be extremely vicious. It drives Pidgey from their nests to eat their eggs. Its evolution, Weavile, is even more devious.
The Larvitar line. Larvitar eats an entire mountain to grow and Pupitar's thrashing can topple a mountain. Tyranitar, too, according to various close-up shots. It's a very dangerous and aggressive Tyrannosaurus rex that can destroy entire landscapes.
While Totodile does not appear to be malevolent, it does not realize the strength of its bite, which is bad because it has a tendency to bite or nip anything. Its first evolution, Croconaw, will not let go when it clamps down and its final evolution, Feraligatr, is surprisingly fast and savagely tears its victim up.
Cofagrigus is said to turn people into mummies.
Genesect. A prehistoric Pokemon. In its time, it was feared as the strongest of hunters. Apparently, Team Plasma makes it even stronger.
Zekrom, the legendary thunder dragon. Although it can be used for good or for evil (like most other species), it can be destructive and chaotic in the wrong handsnote N/Dread, anyone?.
In Resident Evil 4, village leader Bitores Mendez has extremely bloodshot eyes that appear red. One of his eyes is a prosthetic, in fact. A straighter example is the cutscene where a Ganado's eyes glow red before the Plaga bursts out of its head.
In Robopon, in the second game it's revealed that Dr. Zero's visible eye is red.
Pretty much is a warning in "Shadow of the Colossus." The giants' eyes are blue when they're just moving along, but if you see them turn red, it means you either attacked them or they're about to attack you—so watch out!
The various human forms of Lucifer (Louis in his male incarnations) in the Shin Megami Tensei series should partially qualify. One of his eyes are red. And it really is just a warning, given Lucifer's status as more "chaos" than "evil", but it's a distinctive trait worth note.
Once you've earned the True Demon ending in Nocturne, your character's eyes turn red.
The epynomous endboss of Skullgirls has these, in addition to markings on her irises that make her pupils appear like the eye sockets of skulls. Aside from her, the trope is played around with with the rest of the cast: Double's Eldritch Abomination form has red eyes befitting of her serving the Skullgirl, while Filia has red eyes, yet is mild-mannered. Painwheel has glowing red eyes as well, but that's largely an effect of her synthetic Skullgirl blood; she's less a villain and more a victim of mind control. In addition, Valentine has red eyes and cross-shaped pupils. Then there's Peacock, whose mechanical eyes are also red, and she's verymucheager to Paint the Town Red.
In the Soul Series, most notably the first installment, Soul Edge, Hwang in his "no-input " ending returns home, but with devilish red eyes due to prolonged exposure to Soul Edge itself, not forgetting to mention his possessed facial expression that accompanies them well...
In Splinter Cell Conviction, the new batch of Splinter Cells have red optics for their sonar goggles.
In Star Fox Adventures, the four dinosaurs that pace along each side of the Walled City's central structure and attack you if you draw near, and the one huge dinosaur underground that serves as the boss for that part of the mission, are known collectively as "RedEye".
Colette Brunel from Tales of Symphonia gains red eyes when she loses her soul as part of the World Regeneration. She's not so nice when she doesn't have a soul.
Kratos Aurion of the same game has them all the time. It could be because he's a Badass, it could be because he's The Mole to start with, or it might just be a case of Curtains Match the Window, since his hair is the exact same shade.
Emil Castagnier's eyes go from Green to Red when he activates his Superpowered Evil Side, Ratatosk Mode, in the sequel. However, this is a subversion, as Ratatosk Mode Emil is more of an Anti-Hero than anything. Until he remembers that he wants to kill all humanity.
If you follow a (hard to find and easy to miss) sidequest, you learn that his eyes aren't naturally red - they turned red as a side-effect of using magical phlebotinum on himself to strongly boost the power of his spells, since his eyes were where his ability to use magic was concentrated. The Scary Shiny Glasses act as a limiting mechanism, preventing all the energy in his eyes from destroying himself and whatever happens to be around him. Red Eyes, Take Warning, indeed.
In Tales of Graces, it was more of a "Red eye take warning". (Note the singular) You'll notice that Richard has one red eye - Which is a sign that he's possessed by Lambda. This occasionally glows, but doesn't become pronounced until much later in the game when it's rather obvious that Richard is evil. However, during the events of Lineage, Asbel becomes "Possessed" by Lambda, but instead his other eye is purple - signifying that Lambda's become more of a symbiote than a possessor like he was to Richard.
ÜberCharged members of the BLU team in Team Fortress 2 will have glowing red eyes and be completely invulnerable to enemy fire for a brief period.
Tekken 6, Scenario Campaign: Alisa's eyes turn red when Jin disengages her Safe Mode. Main result: She's no longer able to disobey any command Jin wishes her to follow. Like kill Lars.
Devil Jin, the form that Jin transforms into whenever he is fully corrupted by the Devil gene, has glowing red eyes and is not a very nice guy.
Jin's father, Kazuya, has a glowing red left eye while his right eye is normal. His role varies from anti-hero to outright villain to anywhere inbetween at different points in the series.
All vampires in Vampires Dawn gain red eyes upon transformation. The red part increases in size if they are possessed by Shadow Ghosts.
For the orcs in Warcraft, it's one of their defining traits. Drinking the blood of Mannoroth gave them red eyes that glow fiery red when they descend into demonic bloodlust. Grom Hellscream in particular had them for most of his life. When he gives his life to defeat Mannoroth (freeing himself and the rest of the orcs from the Burning Legion), his eyes become clear again.
In Wadanohara, there's an adorable fish with big red eyes. His name is Sal. While he seems innocent enough, he ends up being the main villain of the game who was behind everything.
In The World Ends with You, red eyes are an indication that the person is under the influence of Instrumentality. Only the Reapers actually attack, but still, that it's happening to ordinary people is a sign that something's definitely going wrong with Shibuya...
In Cave Story, Ravils (Rabid Mimiga) have red eyes. King is a subversion.
One of Ballos' eyes is red too, implying the red gem on the demon crown was his.
The dragonslayer's armor in Dra Koi has glowing red eyes, suitable for its role as the sort-of antagonist.
Not to mention Ilya, who uses Mystic Eyes to immobilize and kidnap the hero (ultimately, though, she's not evil).
Caster's use of Mind-Control Eyes is an interesting variant — the whites of the victim's eyes turn red.
Lancer has red eyes too, but he isn't evil in most of the storylines, although he initially appears as such.
Likewise, his master, Bazett Fraga McRemitz from Fate/Hollow Ataraxia, has naturally red eyes, and she's one of the good guys.
The primary theme regarding this appears to be that Servants who possess red eyes typically have varying levels of godhood. For example, Gilgamesh is 2/3 god and Lancer's true identity is a demigod as well. This also appears to apply to Berserker, who is heterochromatic, with one of his eyes being red. However, that may be the result of the Mad-Enhancement that is part and parcel to the berserker class. It is also worth mentioning that Berserker's true identity is also part-god.
In general, vampires in the Nasuverse tend to have red eyes. People who are turned, like Sion and Satsuki, had their eyes turn red.
Inverted in Suika. Shinigami have red eyes, but are exclusively the kindest and gentlest people who have died.
Boomstick: Metal Man was specifically made for killing things. You could tell, 'cause he's got evil red eyes.
Maxwell Edison, in an almost disturbingly well-done Flash video of The Beatles' "Maxwell's Silver Hammer", is depicted with red eyes.
Yang Xiao Long of RWBY typically has light purple eyes, but in battle, her eyes turn red when she's in her Super Mode. Fittingly, she's also a fire user, meaning she can bring explosions when the trope comes into effect.
In Adventurers!, Khrima has solid red eyes. It's an impressive feat of comic-villainy that this makes him look more silly than intimidating.
The "tainted" or Ver'drowendar from Drowtales have red irises; they "have been seeded by a demonic entity". All drow naturally have red pupils, but this is the result of eyeshine and is an evolutionary adaptation to living underground, but results in tainted characters having red on red eyes.
The yellow salamander/lizard-thing. It may not have been evil, per se, but it was certainly dangerous. The lizard was a house fire that killed a small child and his family.
Zimmy plays an odd variation of the trope. Normally she seems to have Black Eyes of Evil, but when she is temporarily purified from the etheric miasma that pollutes her spirit by natural rain, it turns out that her eyes are bright crimson underneath. In this condition she is a lot more coherent and amiable than her usual self, but acts as a reminder that she's not exactly normal, either way.
Hetty has red eyes and is as evil as they imply, as seen from her suggestion that Reynardine kill Antimony and her Irrational Hatred toward Adam.
Sabine's eyes glow red in her true form. There is an early warning of her true nature in this strip, which is before she's revealed to be a fiend. Later, the glowing-red eyes are a hint for the reader that she's shapechanged.
Sluggy Freelance: In one story, Bun-bun the evil mini-lop rabbit is stuck with the annoying animated shadow of the groundhog who's the Anthropomorphic Personification of Groundhog's Day. Unable to attack the insubstantial creature, he's forced to suffer all its antics. But finally it manages to do something that angers Bun-bun so much that he finally snaps. He gets so angry his eyes turn red, and subsequently he emits a red aura. Insofar as there is an explanation to this, it could be that by that time Bun-bun was already a mildly supernatural being due to being saddled with being the Easter Bunny as well as the official "groundhog".
Shadow: What do you want to do?
Kiki the ferret: I have a pretend karaoke machine made from a toilet-paper roll!
Shadow: You like karaoke too?
Kiki: I LOVE karaoke!
Shadow: A one and a two and a
Bun-bun: That's IT! Everybody dies!
Dr. Schtein of String Theory has prosthetic eyes that glow a sinister red, having lost his original eyes in a lab explosion. Thus far, they seem to be foreshadowing his eventual rise to supervillany.They even glow in the dark, much to the horror of his prison roommate.
The Joker has nearly completely red eyes (but yellow pupils). His Batman Beyond: Return of the Joker design (also used for Static Shock and Justice League) also has red pupils sometimes, if you look closely. The red eyes on the latter-day DCAU design, along with the slicked-back hair sported by the Future Joker design in Return of the Joker, were inspired by the novel's descriptions of Hannibal Lecter.
The Batman also gives red eyes to Mr. Freeze (who appropriately has Icy Blue Eyes in other incarnations, but usually hides them behind red goggles, justifying the change made in the cartoon).
Several of Danny Phantom's enemies have red eyes, including his arch-rival, Vlad Plasmius (whose eyes are blue in human form and completely red and pupil-less in ghost-form), and his sociopathic alternate-future self, Dark Danny (AKA Dan Phantom). Red-eyed Dungeon Master Clockwork, on the other hand, despite being originally portrayed as a villain, is good-hearted but on the neutral side.
Danny's own red eyes when under the control of Freakshow in "Control Freaks".
Played With on Gargoyles;Demona, the evil gargoyle, has eyes that glow red when she's angry while our heroes' glow white. However, it was later established that it was a sex-linked trait; Angela, a hero, also has red-glowing eyes.
Odd variant: while Serpentor from G.I. Joe: A Real American Hero was always evil, his eyes briefly turned red when he held the battleax of Vlad the Impaler, one of the many historical tyrants whose DNA he shared. This was presumably a hint that Vlad had really been a vampire in the show's universe and contact with one of his artifacts had evoked that trait in Serpentor.
"Monster in the Monastery". The fake yeti (who are malicious enemy agents) have red eyes.
"The Robot Spy". The titular opponent - which was sent by the Diabolical Mastermind Doctor Zin - has one large red eye.
The Biskit twins in Littlest Pet Shop (2012) have reddish eyes, though they're not so much evil as they are sheltered and distant from those around them.
Dick Knubbler's robotic eyes turn red when he gets pissed off in Metalocalypse. Not so much "evil" as "Jerkass", but when one works for five Chaotic Neutral musicians that cause destruction wherever they go (by accident, sometimes, even!), it's hard to call oneself "good".
Brother Blood, Big Bad of the third season, has eyes that glow red when he uses his Psychic Powers; in the fifth season, the mechanical eyes on the skull-"face" on the Brain's life-support pod also glow red when the Brain is interfacing with his computers or is just angry.
Nearly all the Decepticons of the original Transformers. Later versions had a bit more optical variety.
Transformers Animated has all red-eyed (or in the case of Soundwave, red-sunglassed) Decepticons like the original cartoon, with the exception of the technically-not-a-Decepticon Swindle, who has purple eyes. However, Sari Sumdac, a human child, also appears to have red eyes. They turned blue◊ after she "upgraded" herself. Also, in Animated, Wreck-Gar's eyes turn red when he briefly became a Decepticon due to his Funny Schizophrenia and being incredibly suggestible. An interesting example occurs in Wasp. Originally, he had blue eyes, but after his stay in prison (and subsequent madness and desire to turn Bumblebee into iron filings), they become purple. And what color do you get when you mix red and blue?
The Autobots' eyes also turned red when they were brainwashed by Soundwave into being evil.
Silverbolt from Beast Wars' optics are normally gold, but when Blackarachnia was killed and he uttered Tarantula's name in a vengeful manner, theyturnedred.
Shattered Glass, of course, reverses this, with the Decepticons mostly having blue optics and the Autobots mostly having red ones. Though there are a fair number of green, yellow, and other oddball optic colors on both sides, and Heatwave strangely has red optics despite being a nice guy, even by his fellow Decepticons' standards. Turns out, it's because he's not actually from the SG universe at all.