Agito, upon his introduction, was a cruel and sadistic killer thanks to a horrible childhood. Just as he starts getting better about it, a split personality called Lind shows up.note It's his third personality, and doesn't exactly come out of the blue At his most tame, Lind is twice as blood-thirsty as Agito was at his most wild.
There's also an Storm Rider called Gabishi who functions as an assassin. He doesn't think a job is done unless he's torn the skin from his target's face. Despite being an assassin, he does not kill, thanks to a code of strict laws he follows, but his attacks still incapacitate targets for months, and they have to live without a face.
In Angel Densetsu, while Ryoko and Ikuno usually aren'tthat dangerous, being always around Kitano is not a good comparison for them. There's even a nice Lampshade Hanging, in which Ryoko ends up beating up several times the same guys: she tries to calm them, they get even more scared, she gets annoyed, and then she 1-hit KO's them again.
Lithuania is generally the victim of the cruelty, as he is Russia's "favorite." Russia certainly does count, but truthfully only when the true side of his bipolarity shows, as he is generally quite docile.
Ladd Russo and Graham Spector. Ladd is a token Psycho for Hire who takes maniacal glee in killing anyone and everyone, once beating a man to death by brutally punching him 47 times in the face, even after he died, and expressing his love for his fiancée Lua by passionately threatening to murder her. Graham is a destruction obsessed maniac who expresses himself by dismantling everything around, including people (he also holds deep admiration for the aforementioned Ladd, referring to him as an "older brother"). Quartes outdoes them both in violence. In fact, everyone has shades of this.
Many a villain from Berserk Femto, (or Griffith) less so, and Guts's evil side (Guts himself, too).
Alan Gabriel from The Big O, who, due to him often descending into fits of insane laughter, and him wearing make-up, powdering his face white and wearing red lipstick, is a blatant nod to the Joker of Batman fame. Most of that show is a blatant nod to Batman. R-D also fits this trope.
Grell Sutcliff from Black Butler is Ax Crazy, although somewhat manageable in the anime, at least. In the manga, it takes a superior Reaper to keep her from slaughtering the protagonists, and she's therefore a much less frequently recurring character.
Revy, a Sociopathic Hero, is usually able to keep her crazy under control and her guns aimed at the right people, but in one arc that had her and Dutch storming a ship full of Neo-Nazis, the "Whitman Fever" (named for Charles Whitman, the infamous sniper who went on a mass murder spree from the top of a university tower in Texas) took hold of her, and the only thing that prevented her from killing the non-combatants aboard the ship as well as her Nazi quarry was Dutch's intervention. And in the manga version of that arc, she murdered them all before Dutch got there, with the Charles Whitman speech occurring when Dutch confronted her about it.
The Vampire Twins, Hansel and Gretel. Hansel even carries an ax. They make Revy seem almost normal, fer cryin' out loud. The most unfortunate part of that story arc is they seemed to fool viewers into thinking Rock might turn Gretel around...
Shenhua and Sawyer the Cleaner. Both of them loved to chop up victims with their kukuri knives and chainsaw respectively. In fact Sawyer loved it so much she became a professional cleaner so she could slice up all the unfortunate people that pissed off the different mafia groups in Roanapur. Although amusingly enough, Shenhua and Sawyer are actually two of the only three mercenaries who are sympathetic in the Greenback Jane arc (the third being Rotten the Wizard). Sawyer herself actually becomes a bit of a Woobie once her voice box is knocked away, going into a bit of a Villainous BSOD.
Gennai Doma from Black Lion is extremely murderous; he'll slaughter any ninja in his path. This is because a group of them killed his wife and he got cyborgenetic implants that gave him super strength and other powers.
His arrancar counterpart, fellow Mad Scientist Szayel Aporro Granz, fits as well. He even represents 'madness' as his aspect of death among the Espada.
Ichigo's inner Hollow, Grimmjow, and Nnoitra. Especially Nnoitra, him being Kenpachi's Evil Counterpart and all.
Unohana Retsu's true nature is revealed to be this. It makes sense, considering that she was the original Kenpachi. In fact, she's considered to be the most diabolical criminal in all of Soul Society history — which means that she was a worse criminal than Aizen.
Gettou Mokoto from Change 123 suffers from Dissociative Identity Disorder. Three of her personalities (Hibiki, Fujiko, and Mikiri) aren't actively dangerous to those around them and only fight as a defense (admittedly, Hibiki has a liberal definition of same). It's Zero, her fourth personality, that is dangerous to everyone around her, whether they be enemies, one of her fathers (long story), friends, or even her Love Interest Kosukegawa.
In Chonchu, most characters have their Ax Crazy moment, even non-fighters. It helps that the action happens in wartime and features many ethnic conflicts.
Most of the Awakened Beings are like this in Claymore, due to their penchant for innards, but of the Claymores themselves, Ophelia deserves a special mention for being absolutely batshit insane and willing to fight people on her own side, as Clare finds out. There's a reason why Miria warns Clare about her, and why one of her nicknames is 'the blood-soaked warrior'.
Luciano Bradley, the Knight of Ten, is known as the "Vampire of Britannia" in and is a self-proclaimed "homicide genius". When Zero attacks Tokyo directly, he immediately asks to be assigned to the defense just because he wants to kill people.
Jeremiah Gottwald spends a brief period like this near the end of season one. After hearing Zero's voice over a broadcast channel, he goes completely berserk, steals a mech, and rushes off in an Unstoppable Rage, taking out anything and everything in his way to get to the Gawain and losing his shit even more when Zero insults him. He's calmed down considerably by his next appearance, though.
Cowboy Bebop had two characters who shared this trait. The first and most obvious is Pierre Le Fou who is a complete sociopathic serial killer who just...kills. And has the mind of a 4-year old to boot. The second is Vicious. His megalomania and quest for power leads him sacrifice everyone from his pet bird to old friends with whom he shared many memories. Both share the characteristic of killing for the sake of killing more people.
Death Note: Pretty much all the Kiras qualify when they're at their worst, but several examples stand out:
Teru "Sakujo" Mikami. He loves passing "God's judgment" on the people he kills. During the warehouse scene he had what would be hard to describe as anything but an orgasm after he wrote the SPK members' names in the Death Note. But as it turns out, Near replaced the pages of Mikami's Death Note so that he could not kill them no matter how many times he wrote their names down.
Misa Amane probably counts. She's a bit calmer than your usual Ax Crazy, but she believes that shinigami dying because they killed a person who would have killed their love interest is "a wonderfully romantic way to kill." Also, she was willing to kill her friend without thinking twice for a guy she just met. And unlike the series' other Villain Protagonists, she makes no effort to rationalize her actions.
Dr. Kazutaka Muraki from Descendants of Darkness is looking to bring back his dead older brother Saki. Not crazy enough? His brother killed both their parents. Still not crazy enough? His brother tried to kill him but was stopped (lethally) by the family butler and now keeps his bro's head in a jar till he can revive him and kill him properly. STILL not crazy enough? He's been known to remove his own major organs just to mess with people... and he's a Depraved Bisexual to boot.
Tick Tock in Dorothy of Oz could possibly count as one of these due to her tendency to cut off the arms of those who displease her or give her false information. The fact that she is nothing short of proper and polite before attempting to do this is rather disconcerting. However, she is a robot who is programmed to do whatever it takes to achieve her goals.
A better example would perhaps be Shine, who had no qualms whatsoever about blowing up a city full of innocent people for the sole purpose of traumatizing Mara...and then grinning about it as she screams in pain.
Majin Buu from Dragon Ball Z is this taken to Omnicidal Maniac extremes. Every time he transforms into a new form, he becomes progressively more evil and psychopathic to the point where he just blows up planets without a second thought. While his Power Level in his final form is the subject of much debate among Dragon Ball fans, everyone can agree that said form was definitely the most Ax Crazy. To put it in perspective, the first thing he did after reverting to Kid Buu was try to blow up the planet. When he was foiled, he just made the next shot ten times bigger so he wouldn't miss.
There's also General Blue from Dragon Ball, seeing how he states after executing a soldier for picking his nose that he loves the sound of gunfire executing soldiers.
Demon King Piccolo and his offspring Piccolo Jr., before his Heel-Face Turn.
Nappa as well. He's a complete Hair-Trigger Temper, and nearly loses his sanity while fighting Goku.
Frieza definitely qualifies too, even if it's not as obvious as Majin Buu's later forms. For one thing, he's incredibly sociopathic, and at times, is shown to be extremely sadistic to the point that killing and causing pain seems to be his favourite sport. If you want an example, look no further than the time he destroyed Planet Vegeta and left the Saiyan race almost extinct while laughing his head off and commenting on how wonderful and beautiful it was.
Any Diclonius from Elfen Lied has the potential to be extremely Ax Crazy, although the show manages to raise a lot of sympathy for them as well. Lucy, the primary character of the series (when she's not in her cute and harmless "Nyuu" mode, or back when she was just Kaede prior to becoming a genocidal lunatic) is one of the most frightening characters you will ever meet. Her introduction scene has her violently slaughtering people left and right with her vectors at a lab while wearing nothing but a helmet.
Agon Kongo was implied to be this early on in the series. After being defeated by Deimon, his earlier axe craziness evolves into him simply being incredibly selfish and willing to use violence to further his goals if he can get away with it. His later characterization of just being a major prick means his earlier actions could be seen as him just being a ridiculously petty jerkass.
Hiruma purposely builds his image off of being seen as one. Once you get to know him, you realize there's actually meaning to his madness.
Similarly, Gao seems Ax Crazy, but he's actually capable of controlling himself, which just makes him even more frightening as he can't be manipulated.
Barry the Chopper is a butcher turned serial killer who enjoyed chopping up flesh, but grew bored of just doing it to meat and started hunting humans instead. His eventual execution by hanging was well-publicized, but in truth, like many other Death Row inmates, he was instead used in human alchemical experiments where his soul was ripped from his still-living body and bound to a suit of armor.
Sakon Daimaru from Gamaran when he fights seriously. Partially justified by the massive trauma caused by his ambitious big brother Ryuugo. Many of the Muhou Ryuu's warriors tend to be nutjobs, with Muraku Matsumoto standing out above them.
Several characters (more like almost all of them!) from Getter Robo, most notably Hayato as a delinquent (or adult terrorist in some adaptations) and Ryoma, who actually does use an axe. Appropriately, Ryoma's Humongous Mecha uses a Big Fraggin' Axe too and it goes Ax Crazy as well, becoming sentient and going on a rampage that would make EVA-01 wet itself in terror.
Shogun the gelada baboon from the Ginga Densetsu Weed manga. Literally in this case since his main weapon is an axe. Also Hougen.
Takamura and Ryo Mashiba have their moments too. Mashiba himself goes mad when he fights Sawamura, which results in one of the dirtiest matches in the whole series.
Alucard from Hellsing. Sadistic, bloodthirsty, clearly off his rocker, and nearly godlike in power. Also one of the series' most popular characters... maybe due to his Undying Loyalty to Integra and places he won't go. In the anime, less so.
A solid chunk of the Vatican's IscariotSectionXIII qualify as well. Father Alexander Anderson is a Noble Demon and Holy Hitman who becomes absolutely bloodthirsty whenever he encounters Alucard or another vampire/Nazi/heathen/whatever else happens to pop up. His Action Girl protégés, Heinkel and Yumie, aren't much better than their mentor, although no one's as nuts or off their gourd as Maxwell, who takes it so far against the innocent population of London that Anderson has to kill him.
Suikotsu's evil side from InuYasha. Also Jakotsu, the local Depraved Homosexual. He has an attraction to Inuyasha and in his sadistic version of love tries to torture him to death because of it.
Many of the villains from JoJo's Bizarre Adventure, like the vampire Jack the Ripper, Gray Fly, Devo the Cursed, Alessi, etc., who enjoy causing destruction, killing people, and/or creating general mayhem of varying degrees. And all of them combined aren't nearly as bad as Dio Brando. The fact that they all bow down to him just serves to highlight his evilness.
Hibari (more Tonfa Crazy though), Mukuro, Byakuran, Birds, Belphegor, Rasiel, Glo, Xinia, Xanxus, and Daemon Spade from Katekyo Hitman Reborn!.
Kekkaishi gives us Kaguro, who starts out power hungry and then spirals into full-blown ax-crazy. Made even worse in his Ayakashi form since he has the ability to sprout a limitless supply of swords from his body, making him impossible to disarm without killing him.
Captain Continental of Legend of the Blue Wolves. He tries to sexually subdue subordinates that take his fancy and if they happen to refuse him he whips them and beats them half to death unless they submit. And if even that doesn't work he just rapes them, beats them, and forces them to participate in various sexual activities with absolutely no remorse. In fact, he seems to get sexual pleasure (if his constant Slasher Smile is any indication) out of breaking, beating up, and raping people. And at one point, he ran at Leonard, all the while screaming like a savage when Leonard tried to stop him from continuing his No-Holds-Barred Beatdown on Jonathan.
In this series, most evil characters were at least cold or sensible, and at the most just a bit odd. That is till Tsukuyomi, deprived of seeing her sempai Setsuna for months turned from merely fight-happy with a touch of blood-lust to a complete Psycho Lesbian for her rival swordswoman.
In Mirai Nikki (The Future Diary), the character Yuno Gasai uses axes (along with many other sharp weapons) to kill people that threaten her relationship with the protagonist Yukiteru Amano (this relationship is mostly one-sided as she's a Stalker with a Crush).
This sums up the majority of the cast really. Even Yuki has a bit.
Monster: Johan Liebert is an extremely subdued version of this. He never goes visibly nuts, is very soft-spoken, polite, and overall calm at almost all times. However, if you associate with him, you will die. If you meet him, you will die. If you make eye contact, you will die. If you so much as think about him, you will die. And if you won't think of him, he just has to think about you, and you'll be just as dead.
Deidara is also pretty crazy as evidenced when he tried to kill both himself and Sasuke via blowing them up.
Orochimaru and his Sound 4 squad display various levels of this trope. Ukon and Sakon, in particular, like to play with their victims.
Sasuke displayed these tendencies after getting Orochimaru's curse seal and attempting to leave Konoha, but it's from chapter 480 onwards where he really falls into this trope due to Sanity Slippage and Madara's influence. As if deciding to kill every last person in Konoha wasn't enough he's been reduced to a homicidal maniac who will kill anyone if they become the least bit inconvenient to him. Danzo's Shut Up, Hannibal! didn't exactly help, either.
Nine-Tails, AKA Kurama, is a monster whenever he's angry- and a sociopath on the rare occasions he's happy. At least, until Naruto trounced his ass, swiped some of his chakra, and then befriended him, taking the edge off his personality and changing him into a trusting ally of his host.
Shogouki ("Unit 01") itself is prone to spontaneous acts of horrific violence and cannibalism against Angels and fellow Evas when it slips into Berserker Mode.
There's the Dummy Plug System, which is powered by the seemingly docile Ayanami Rei.
Asuka is a hazard to the health of anyone or anything that isn't named "Kaji" or "Hikari" if they push her buttons. This effect is turned Up to Eleven if Shinji is the victim or if she is in Nigouki ("Unit 02"), and passes the horror threshold when she has to fight 9 Evangelions at once.
To illustrate just how ax crazy she had become by the end, she holds an enemy Evangelion above her head, cracks it open, and bathes in its blood with a sadistic grin on her face. But the girl had already been through personal childhood hell and pushed over the edge, thanks to the mortifying Mind Rape she got from Arael, and she was ready to wreak havoc on the world around her as a way to curb her pent up angst and exact a little revenge on her enemies.
And then there's Shinji himself whenever he gets angry, breaks down, or gets significantly stressed. He's by far the worst culprit, because when he had a chance to wish for paradise in Lillith's egg, he wished for everyone in the world to die- and it happened. He also cleaned his weapon to Asuka's boobies out of perverted desire while she was in a coma from starving herself to death, possibly even slitting her wrists, andthrottled her with intent to kill when she shunned him during Instrumentality.
Asuka's mother Kyouko after piloting EVA 02 and having a part of her soul sucked into it. The half stuck in the EVA was the kind, loving side. The half left in her body turned deranged and psychotic, which led to her hanging herself with a doll she believed was Asuka.
Mizuki Asahina and Hiruko in Nightmare Inspector say that dangerous sleepwalkers running into Ginseikan is a common occurrence.
Kuro won't think twice about killing to further his plans, and tried to kill his pirate crew and a young Ill Girl to wipe out any trace of his old life.
Shilew of the Rain was once a guard at Impel Down, but his love of killing prisoners eventually leads him into jail himself. The first thing he does when he gets out after Magellan authorizes his release to deal with Blackbeard is to kill all guards that come his way.
Eneru wants to knock his Floating Continent homeland out of the sky, and already did this to the island of his birth. Out of them all, he's probably the most legitimately insane.
Probably the worst of the lot is RobLucci, who was sent in at age 13 to deal with a hostage situation where 500 soldiers were captured by pirates. He kills everybody involved.
No, no. In a series loaded with violent psychopaths, Akainu stands out among them all. His first real onscreen appearance has him ordering the destruction of a ship full of innocents because there might be a couple scholars aboard; from there, he only gets crazier and crazier, responding to every single threat with absolutely staggering force, regardless of the collateral damage it causes. We finally get to see just how truly nuts he is, however, when he responds to Coby's call for the violence to stop by immediately trying to kill him. While Eneru is definitely crazier, he's nowhere near as unstable as Akainu.
Bartolomeo. He is more anti-social than Eustass Kid, he drops a fake publicum to the audience in his first battle, though, it is known that he has bombed innocent citizens and grilled pirates on a skewer and broadcoasted it. He even cuts the tongue of a noble off, just because that person talks shit about Luffy. That's right. This crazy guy is a LuffyFanboy since the Logue Town arc.
Please Save My Earth has two characters that qualify, Rin and Shion, and the best part is that they share a body, or rather that one is the reincarnation of the other absorbing the reincarnation's personality. It can get a little confusing...
Best expressions of crazyness on the main characters would be Oz when he is usurping Alice's power.....he makes this face◊.
Lacie. How crazy is she? She kills two men that were threatening her and Jack and dances and sings in their blood.
From the main series: Kyouko and Sayaka have various shades of Ax-Craziness at various points in the story. Sayaka especially goes psycho as the series goes on, eventually becoming a witch and having to be put down. Yeah, it's that kind of story.
The manga makes it worse, just with some of the facialexpressions at some points. Probably helps that the manga was released alongside the anime and so the artist knew little more than the audience at the time of drawing it, so not knowing who was really good and who was really bad, she apparently just decided to make everyone insane... which isn't such a bad thing, really.
Fanon often makes the various characters of Ranma ½ into this... which is actually a fairly easy leap to make, given that Comedic Sociopathy is a common mode of humor and all of the characters are JerkAsses, with or without a golden heart to soften things. Ryoga enters the series by ambushing Ranma from out of nowhere, apparently intent on viciously hurting Ranma... over bread. Shampoo chases Ranma across China seeking to kill "her" in order to avenge her lost honor, then is willing to consider killing her rivals for his hand. Mousse not only routinely tries to kill Ranma because Shampoo isn't in love with Mousse, but has no qualms about hurling volleys of bladed objects in all directions... in the middle of crowds and on public streets. Ranma's mother, Nodoka Saotome, wants desperately to meet the child who grew up without her... yet is so dedicated to upholding an immorally vague Seppuku contract that she carries a sword with her wherever she goes, and is almost frightening in her eagerness to pull it out.
Tsukune's Ghoul form definitely counts, what with all the bloodshed, insane laughter, and repeated use of the word 'kill'.
Akuha/Aqua Shuzen of the second manga season is shaping up to be one of these due to being quite a Yanderefor Moka. She has a very dark and violent side, and will annihilate anyone who gets in her way or pisses her off. She goes as far as to threaten the lives of Tsukune and his friends when they try to take Moka from her and completely looses her shit when she sees Moka and Tsukune hug.
The three brothers after Mugen in the final episodes of Samurai Champloo. Every one of the three (except possibly the eldest, Toube) is a complete maniac; Denkibou found it impossible to contain himself once Mugen was in sight, and Umanosuke is a sadistic bastard with several Kick the Dog moments in a row.
About ninety percent of the cast of Samurai Deeper Kyo, including and perhaps especially the main character.
Sekai and Kotonoha, two almost-protagonists from School Days. Especially Kotonoha (in the anime and game).
Mayuka, in the second Tenchi Muyo! movie, after Yuzuha pushes her berserk button. Sasami's love vibrations break her out of it however, and she gets better. Unfortunately, Yuzuha does not. As Tenchi's a twit.
Yuzuha herself also fits the bill, as the only thing that would've stopped her misaimed vengeance fueled rampage was the only thing nobody thought to give her. You'd think Ryoko at least would have figured this out with how often she surprises everyone else with Marshmallow Hell when drunk. Perhaps they should have given her a shot of whiskey before heading to Yuzuha's dimension.
Chiri Kitsu from Sayonara, Zetsubou-Sensei has been Flanderized into this. Originally she'd get extremely annoyed and occasionally violent if things weren't done properly, then in the later episodes she would regularly beat people with a shovel for doing things improperly, and in the latest season seem to have dropped all pretexts and become generally Ax Crazy.
Soul Eater has several characters like this. Crona and Maka act this way under the influence of the black blood. When given the opportunity, Stein counts as this even without the influence of the Corruption.
After he realizes that Ichigo loves Masaya and not him, Kisshu from Tokyo Mew Mew becomes this way via Love Makes You Crazy. In Episode 45, after Ichigo refuses to leave Earth with him, he goes into a rage and tells Ichigo that if he can't have her then he'll kill her. He even employs the usage of a Slasher Smile.
Under Grand Hotel: Swordfish, on account of him being such a Yandere for Sen. He threatens anyone who expresses an interest in Sen with death, has sex with Sen to the point where it nearly kills Sen, tries to strangle him, kills the guys who raped Sen saying that he did it for Sen's sake even after Sen told him not to kill them, moves out of Sen's room and lets a rapist move in when angry with Sen, and slices the throat of another one of Sen's rapists right in front of him while saying "I love you, Sen." This being after he tried to get Sen to kill the guy himself but Sen refused.
In Vagabond by Takehiko Inoue, Chapter 162 (Volume 18) reveals that in the aftermath of the battle of Sekigahara, the future Miyamoto Musashi (then Shinmen Takezou) is rather Ax Crazy. Otherwise though, he avoids the trope and the regular portions of the story had shown that Musashi had long grownout of this.
Thorkell the Tall from Vinland Saga butchers a bunch of people he's been chasing for trying to surrender. With a pair of great axes no less. He's also known for killing his own men for being either cowards, or disrespectful to his enemies.
Witchblade: Shiori, one of the Cloneblades, becomes Ax Crazy after fighting Masane Amaha. After going off the deep end, she murders a nurse, has sex with a corpse, and tries to rape Takayama, the protagonist's boss.
Kiryu and the other Dark Signers from Yu Gi Oh 5 DS as well as Luccanio and his fusion form Aporia.
Yu Gi Oh Zexal has a couple, including Tron, IV, and the Big Bad Dr. Faker or so we are led to believe. Yuma even has an ally that fits, his former would-be Yandere Anna Kozuki. (This is not by Yuma's choice, I should add. Anna just tends to show up and insist on helping him, and seeing as she has a BFG with her most of the time, it's kind of hard to tell her to go away...)
And, of course, there's Vector, who's nearly on the same scale as Hannibal Lector...
YuYu Hakusho: Shinobu Sensui, and the whole of The Sensui Seven for that matter fill in the shoes something good. Although Itsuki doesn't just take the cake, he takes all the cakes.