Galinda: What is this feeling so sudden and new Elphaba: I felt the moment I laid eyes on you Galinda: My pulse is rushing Elphaba: My head is reeling Galinda: My face is flushing Both: What is this feeling? / Fervid as a flame, / Does it have a name? / Yes! Loathing / Unadulterated loathing
Everyone has heard of Love at First Sight. Two people meet and instantly one or both falls in love. Happiness ensues, except whenit doesn't. On the other hand, sometimes people despise each other from the first moment they meet, often before they've even really had a chance to get to know each other. That is Hate At First Sight. When it's mutual, there will be lots of blatant antagonism. When it's one sided, the target of hatred might be completely confused.
Maybe it's their instincts giving them a hint that they just can't get along with this person or maybe it's just something random and unfounded, but either way one or both characters will simply never get along or at least not without a great deal of trial and tribulation. While you might think that this trope would be particularly common between a hero and a villain, it's actually quite common between two good characters as well.
Compare Love at First Sight. See also Irrational Hatred.
Yozora hates Sena from the moment they meet while Sena wants to be friends, which gradually evolves into an extremely Vitriolic Best Buds type relationship due to Yozora constantly being horrible.
Kobato also hates Sena from the instant they meet and for much better reasons than Yozora: Sena acts like a creepy stalker towards her.
Kobato and Maria subvert it though. They look like they hate each other, but are actually best friends. For Kobato, Maria seems to be the only person she does consider a friend: She avoids all her classmates who want to get along with her.
Izaya and Shizuo in Durarara!! hated each other instantly. Shizuo could tell exactly what kind of person Izayawas while Izaya was constantly unable to manipulate Shizuo due to his hostility. This hatred only grew over the years due to numerous Noodle Incidents during their high school years, and by the time the series starts, there is no hope of any civility between them.
Yotsuba to Yanda in Yotsubato. Yanda tried to enter her house while her dad was away. Yotsuba was taught not to let any strangers in when she's alone. Which leads to her slamming the door on his face. The fact that Yanda enjoys making her angry on purpose doesn't help improve her opinion of him.
In Jojos Bizarre Adventure Dio Brando immediately saw Jonathan Joestar as an enemy the moment he laid eyes on him. Poor Jonathan only wanted to be friends with Dio. It's not until Dio pulls stunts like stealing Jonathan's love interest Erina's first kiss, killing his dog, poisoning his father, and becoming a mass-murdering vampire that Jonathan sees Dio as an enemy. Even then, Jonathan still considers Dio a brother.
In volume 10 and episode 11 of season 2's Ore no Imouto ga Konna ni Kawaii Wake ga Nai!, Ayase and Kuroneko both instantly dislike each other the first time they formally meet. Ayase doesn't like her because Kuroneko broke up with him after dating for a while and breaking his heart, while Kuroneko wonders why Ayase is even visiting him due to the way she tends to treat him like dirt. She also remembers Ayase from an earlier moment when she first found out about Kirino's Closet Geek tendencies, and how unaccepting she was of it initially.
In Beelzebub, Hilda and Aoi Kunieda got along okay at first, though Hilda is pretty cold. They didn't hate each other during their first met, but they developed a rival relationship. However, when Hilda got an amnesia, she became a kind, innocent, sweet and naive girl with an obvious crush on Oga. New Hilda hated Aoi (and Furuichi) at first sight, though.
In Aliens, Ripley immediately detests "artificial person" Bishop upon meeting him in the Marines' mess hall because of her prior experience with a malicious android on board the Nostromo.
In the Lindsay Lohan version of The Parent Trap, Hallie and Annie dislike each other from the moment they first see each other and see they look exactly alike. They start to get along after discovering that they are long lost twins.
From the moment she first laid her eyes on him, Madeline sees only the bad stuff in Pepito, calling him a bad hat. Her hatred is sealed when he scares her whole class later that night, dressed as a demon.
In the first X-Men film, Wolverine and Cyclops show an immediate disdain for one-another before even a single line of dialogue is spoken.
In 'Salem's Lot Susan's mother hates Ben almost immediately and fabricates several reasons why. Ben satisfies himself by knowing he gets along with her father while Susan and her mother have a major fight over it.
Rhea in Wizard and Glass instantly loathed the young and beautiful Susan. Susan tried to clear the initial bad impression, but the antagonism remains through the rest of the story.
Idril disliked and distrusted her cousin Maeglin, without really knowing why, almost as soon as she met him, though she didn't hate him. He fell in love with her and being unable to woo her, became so embittered he eventually betrayed their whole city and got it sacked.
In Harry Potter, Harry dislikes Draco Malfoy from the start. Partly because of his spoiled attitude (which reminds him of his cousin Dudley) and partly because of his lack of sensitivity when Harry mentions he's an orphan. This ends up cemented later when Draco sucks up to Harry on the train after finding out who he is.
In Warrior Cats, Longtail, Sandstorm, and Dustpelt hate Fireheart the moment he joins the Clan, and the feeling is mutual. Eventually Fireheart saves Sandstorm's life and she falls in love with him, Longtail, without the influence of Darkstripe and Tigerstar, eventually becomes loyal to Fireheart (particularly after Fireheart begins to treat him with more respect), and Dustpelt continues to be a bit of a jerk, but in their old age they begin to relish their rivalry more than anything.
Live Action TV
A 3rd Rock from the Sun episode involved Dick getting upset that a cafeteria lady hated him. This is pretty bizarre when you consider how Dick is usually apparently okay with several regular and recurring characters disliking him. In fact, when he got angry that she didn't even hate him for an actual reason, Nina replied "Does she need a reason? 'Cause I can give her reasons."
In Wicked, Elphaba (the Wicked Witch of the West pre-wickedness) and Galinda (that is, early Glinda) sing "What Is This Feeling?", sung almost as if it is a lover's ballad, except instead about love at first sight, it's this.
In Duel Savior Destiny Taiga and his magic teacher Downy loathe each other from the moment they first meet. It's not until the final route that they actually have a conversation that isn't completely hostile.
Lightning towards Snow in the Back Story of Final Fantasy XIII, mainly because the epic clash of theirpersonalities and because he plans to take her precious Serah (her little sister and his fiancee) away from her. It doesn't help any that Snow is generally quite oblivious to this and keeps trying to treat Lightning as a sister-in-law already.
In Fate/stay night Shirou has this reaction at least twice and for similar reasons both times, though he doesn't really consciously recognize it.
He and Archer are instantly in opposition because Shirou recognizes that Archer embodies his ideals as a hero of justice, but absolutely despises his methods. Archer, in turn, views Shirou as a naive fool whose insistence on fair play, honor and trying to help everyone will just get even more people killed and will lead him to being crushed by his ideals like Archer himself, who is Shirou's future self.
He's also instantly hostile to Kotomine Kirei, though in this case Shirou doesn't really try to think about it too much and just calls him a fake priest. In truth, he doesn't really dislike Kotomine, but feels he must oppose him because like Archer he is someone that is just different enough to merit his instant hostilities. He actually likes Kotomine since they're so similar.
Kyousuke despises Usami from the moment they first meet in G-Senjou no Maou, though he does his best to hide it. Her passion for justice, her last name, her slovenly attitude and her nosiness grate against the antihero Kyousuke. That said, she is the main heroine of the story, but Usami has to put up with a lot before that.
Akiha has a tendency to hate people quickly in Tsukihime. She instantly takes a dislike to both Ciel and Arcueid. As far as Ciel goes she has pretty solid reasons: Ciel is a member or an organization that often opposes non human beings such as herself. Arcueid, on the other hand, pisses her off because of how whimsical she is as well as being too close to her brother.
In Amagami Miyu instantly has a great deal of loathing for Morishima Haruka, who finds her to be incredibly cute. At first it looks like this is a measure of jealousy due to Miyu being Miyu, but later arcs show that Miyu is quite supportive of her brother's relationships.
Haley and Crystal in The Order of the Stick met in the thieves guild and within moments had lethal grudges against each other. This cemented Crystal as Haley's rival, meaning she would always be at the same level as Haley or better.
In Homestuck one form of Troll Romance is called kismessitude, a romantic relationship based around hate instead of love. When Karkat sees John for the first time on his computer screen, Karkat instantly develops a "hate-crush" on John, and believes him to be his destined kismesis.
In Two Best Friends Play, while playing The Walking Dead, Matt mentions how much he usually hates child NPCs in video games, and how impressed he is at the writers for making Clementine likable. Then Duck shows up, and Matt's reaction—before Duck even has a chance to speak—is "Oh hell no." For the rest of the playthrough, Matt interacts with Duck as little as possible and laughs at Duck's misfortunes.
During one of the more memorable episodes, a new worker joined the power plant named Frank Grimes, who represented an average person dropped into the world of The Simpsons. He quickly became Homer's Enemy, as noted in the episode title, due to how appalled he was at how lax and incompetent Homer was while still having such an idyllic life, which just keeps getting worse as the episode continues. At the end, his hatred gets the better of him and in a fit of anger grabs raw electrical wires while ranting and is killed instantly. At his funeral, a sleeping Homer asks to change the channel and everyone laughs.
One episode from the 2012-13 season featured Tina Fey as a substitute teacher who kept vindictively insulting and punishing Lisa for no apparent reason at all. She eventually admit the reason being is because she thought Lisa was one of those beautiful popular girls. Lisa however, is happy that someone actually thinks she's popular.