In several of the later Animorphs books she narrated, Rachel expressed fear that she might be slowly becoming like this. Too bad she was this soon after acquiring her grizzly bear form.
In Adam R. Brown's Astral Dawn series, Tyr derives pleasure from battle, death and destruction. The only reason he's involved in the plot to conquer Heaven with Set and Devlyn is because it will give him more opportunities to fight and kill.
Bazil Broketail: Although Norwul is normally quite reasonable and mellow, he tends to be quite aggressive when in a fight, dispatching his enemies in brutal way (as that unnamed rapist in the town of Yazm can attest) and always supporting a direct approach when planning battles.
Even Silk, spy and assassin by profession and normally a Combat Pragmatist of the first water, gets positively giddy when he finds out that one of their enemies is the regional equivalent of a fairly senior ninja, and relishes the thought of testing himself and seeing how good they actually are.
All the Brothers in J.R. Ward's Black Dagger Brotherhood series fit this trope. They're elite vampire warriors bred to protect the vampire race from vampire hunters and they love their job. All of them, but especially Zsadist, live for fighting and killing — until they meet a good woman because Love Redeems.
In Bulldog Drummond, Colonel Drummond enjoyed the war — he used to sneak across enemy lines on unofficial solo attacks for fun — and finds the subsequent peace "incredibly tedious". It's a great relief to him when he discovers a sinister criminal conspiracy that needs taking down.
At the beginning of Monsters of Men, when Mayor Prentiss witnesses an army vastly superior to his own descending upon his city with the intention of killing him in retribution for a genocide that he committed for the sole purpose of instigating this war his reaction is to grin with joy and utter: "War. At last."
In The Chronicles Of Alice Hatcher, formally Nikolas loves fighting, ready to swing his hatchet into anyone unlucky enough to get too close to his loved ones. It's how he got his name.
Lord Borel, in Roger Zelazny's The Chronicles of Amber, has as his main goal in life to outfight either Eric, Corwin, or Benedict, some of the other best swordsmen in that universe. Corwin shows him that it's not all about bladework.
Skeeter Traps from Chronicles Of Magic is a ten-year-old version of this. While one of the good guys, she enjoys fighting and is naturally prone to violence.
Conan the Barbarian is a straight-up example of this trope. He comes from a culture of northern barbarians who consider the thrill of battle to be one of the only true joys in life.
The warships and militarised drones of The Culture all exhibit this to some extent. They have exceptional self control but when they feel that circumstances are such that they can let themselves off the leash they will utterly annihilate their opponents and have enormous fun doing so. In at least a couple of cases, they've been known to keep very high resolution recordings of the battles they're involved in that they seem to get an almost sexual thrill from watching later.
Verde from 'Cursed World' gets a little... Into, fighting and training to become stronger.
In Alan Dean Foster's The Damned novels, all of humanity were Blood Knights. It turns out that the humans only fought each other because they lacked an external enemy to fight. When such an enemy presented itself...
In Cold Days, Harry finds that the Winter Mantle has a very strong drive for this. In fact, after a major battle, Karrin says he had an erection throughout the battle. Suffice it to say, the change very much scares the person.
The Einherjaren are Viking soldiers from Valhalla who love nothing more than a good fight. In Battle Ground when everyone else has the common sense to be at least a little apprehensive as full scale war is breaking out around them, the Einherjaren are absolutely thrilled. When the Jotun arrive, they're so excited that they all but trample each other in the rush to get into the fight.
The Jalis in Deltora Quest are a race (or tribe) of Blood Knights, who relish combat and live for battle.
The main character in Dis Acedia, Shroud, develop into one, but Brina turns out to be outright addicted to warfare.
The Nac Mac Feegle in the novels are clans of "pictsies" who live primarily for drinking, fighting, and stealing, preferably at the same time. Each clan also has a Gonagle or Warrior Poet, though in this case the poetry itself is a weapon of war. Taken to extreme when it's explained that a Feegle left all by himself may very well kick himself in the nose just to keep himself occupied.
There's also the D'regs, a nomadic tribe from the desert regions of Klatch characterized by their love of fighting. It's said the D'reg word for "stranger" is the same as their word for "target". There is one scene in Jingo where someone has a hard time translating the concept of "fighting for freedom", since in the D'reg language they are the same word. It's also mentioned that when the nations of Klatch and Hersheba drew an arbitrary border across the D'Regs home desert, they leapt on this new division of nationality as a wonderful excuse to start fighting each other, too.
Ankh Morpork's Sam Vimes is a Blood Knight whom constantly restrains himself by hunting and arresting criminals as a Watchman to satisfy his hunger for violence. It becomes a plot point in Night Watch and Thud! that Vimes has a vicious nature buried deep that he calls "The Beast" which seeks violence bloodshed but Vimes keeps it in check, he's The Watchman who watches even himself.
In Dragon Bones, Ward mentions this as the reason why his father fought in the king's wars so valiantly, even though he hated the king's guts — he just enjoyed fighting so much. Ward himself gets into the mindset, too, and is rather disturbed by it. He doesn't make it a lifestyle. Averted with his aunt and tactical advisor, Stala, who is strongly opposed to fighting for the sake of fighting, and advises him to only fight if he's clearly stronger (or has a bigger army) than the opponent.
The "Warriors at the End of Time" in some of Michael Moorcock's fantasy novels (particularly The Dragon in the Sword) are said to be powerful soldiers who loved fighting for its own sake, and were then damned to fight on behalf of the Cosmic Balance for all eternity.
David Eddings' The Elenium and The Tamuli trilogies had the Atans, a race who bred themselves as fighters — but did so a little too well, giving them hair-trigger violent reflexes, so that their society was plunged into endless war until they voluntarily pledged their entire race to slavery to the peaceful Tamuli, who as a result gained an unconquerable empire by default. Every Atan believes that they have to have a master to hold them back, or they go on killing sprees.
The Four Horsemen Universe: Tortantulas love to fight, and being Giant Spiders with grasping hands and thorax-mounted eyes that give them incredible peripheral vision, are extremely good at it. They often pair up with Flatars, which ride saddle on the Tortantula into battle.
Fox Demon Cultivation Manual: Ze Chen is so obsessed with fighting that she even barges into Rong Bai and Zhu Qin's fight, which has nothing to do with her, just so she can attack Rong Bai.
Sekhmet, in Matthew Laurence's Freya series, is wholly and completely this, right down to licking the blood of her foes off her hands after massacring them.
Sculley from 1356, who is very upset to realise he hasn't killed anyone in at least a month.
Sculley: Are we fighting yet? Robby: Yes! Sculley: Who? Robby: Everyone! Sculley: About bloody time!
Major Sven Kornie in The Green Berets. He is based on Lauri Törni — see the Real Life section.
Greystone Valley's villain Baelan is a ruthless warlord who has grown frustrated with the fact that he's trapped in a world with nothing worth conquering. The entirety of his villainous plan is focused around opening up infinite other worlds where he can do battle.
Hell most Gryffindors in general appear to be Blood Knights as their eagerness for adventure and glory goes beyond heroics and into Suicidal Overconfidence. Itís especially evident in Final Battle as JK noted the Gryffindors were most ready to fight for the sake of showboating and victory while Hufflepuffs (who were second biggest volunteers) are risking their lives out of genuine desire to protect their friends and loved ones.
Sirus Black is a Nice Guy but he adores a good fight and his angst in fifth book stemmed from his inability to go out and aid The Order of the Phoenix, not helped by Snape mocking him over it.
Mad-Eye Moody one of the greatest Aurors to ever live, personally filled up most of Azkaban with inmates and even past retirement Moody becomes giddy over the idea of bringing down dark wizards again.
Although, as Sirius Black points out, he was one of the few Aurors who refused to use the Unforgivable Curses, preferring to bring his targets in alive: nevertheless, he was still capable of killing in a fight (presumably by other means than an Unforgivable Curse) if he had to, as the Death Eater Evan Rosier found out.
A lot of Death Eaters fall under this as killing, maiming, torturing and destruction is what they excel at. Bellatrix Lestrange and Fenrir Greyback in particular stand out as being the most psychotic even by Death Eater standards.
The Heroes by Joe Abercrombie has two big ones: Bremer Dan Gorst of the Union's entire existence is fighting to the point where he compares fighting one-on-one with another enemy to romantic affection. The other one is Whirrun of Bligh who actually faces off against the former.Gorst is extremely disappointed when it ends prematurely.
History of the Kings of Britain: The Trojans' battle with the Aquitanians is decided by Brutus' ally and fellow Trojan Corineus, who turns the Aquitanians to flight by a reckless charge. As they already retreat, Corineus berates them for running away and calls on them to turn back and fight with him. Later, after settling in Cornwall, Corineus takes "great pleasure" from wrestling with the hostile giants who live there. When Brutus fights a battle with a troop of giants, he makes sure the giant Gogmagog is captured alive only so that Corineus can wrestle him, and Corineus is "delighted by this".
Most of the Career tributes in The Hunger Games, who are trained from childhood to fight and to treat the Games like a game and an honorable tournament. They usually proudly volunteer at the reapings, something most kids from the lesser districts would never consider doing.
Cecily Herondale from The Infernal Devices, is a milder example of this. Gabriel Lightwood notes that she seems to enjoy fighting.
99.99% of the Drow race from R.A. Salvatore's Drizzt novels (and Dungeon & Dragons in general) are Blood Knights. They are born, raised, and die (usually before anything close to the natural end of their 700+ year lifespan) in a perpetual power struggle featuring hundreds of noble houses. And ninety nine percent of them (with the exception of Drizzt, his father, and a couple others from other scenarios, and they partially meet this tropes standards in the sense that although they don't kill for fun they get fun from killing), live for it and enjoy every second of their blood-soaked lives. Even Drizzt is a Blood Knight to a certain, less evil extent: in one of the long introspective commentaries placed at the opening of each act, Drizzt comes to realize that he is a "creature of action": he can only be truly content if he has his swords strapped to his waist and is going somewhere, though this is more of a thirst for adventure than a true hunger for battle. He has, however, been known for bouts of berserker rage when the right buttons are pushed. Drizzt's rage does not hinder his incredible focus in the slightest, and makes him hundreds of times more dangerous to even disciplined foes.
Speaking of R.A. Salvatore's novels: Artemis Entreri. The most Blood Knighty Blood Knight of all Blood Knights!
The most wild battle rager Thibbledorf Pwent! He is a blood knight personified!
The Valerians of the Lensman universe are this — seven foot tall denizens of a three-grav world, whose favourite weapon is the space-axe (because personal shields increase their resistance with the velocity of the impacting body), whose ultimate goal is a warrior's death, and for whom a peaceful death is a disgrace. As Smith himself says: "No bifurcate race, and very few others, have ever willingly faced the Valerians in hand-to-hand combat." Fortunately they are the good guys, and more fortunately they are very good at recognising authority (in terms of who to kill and when to stop).
The Rohirrim in The Lord of the Rings seem to have cultural elements that reflect this, And then all the host of Rohan burst into song, and they sang as they slew, for the joy of battle was upon them. Not to mention when they later charge at the, far numerically superior, enemy screaming "Death!".
While Karsa Orlong mellows out some during the course of the series, he begins it as straight-up obsessed with combat, bloodshed and glory. He arrogantly derides his companion Bairoth Gild for advising caution when they are about to ambush the warriors of a rival tribe and charges straight in. Even as he sees more of the world and learns to think before acting, he never stops charging right into battle for battle's sake. He also wants to end civilization as it is and has no compunctions about the bloodshed that would cause.
We don't see Fener in action onscreen, but conversations about him suggest that he is rather irritable and fond of bloodshed. Not surprising considering he's, you know, the god of war.
A Master of Djinn: Siti is positively gleeful when getting into a fight, enjoying this greatly.
In Musashi, the titular character and his Foil, Sasaki Kojiro are both this. However, Musashi fights in order to test himself and become a master, while Kojiro has a sadistic streak, and fights to promote his name.
Hunter from Neverwhere. She wants one thing — to fight and kill the most ferocious beasts. She takes pleasure in this.
In 1984, this is one of the things Big Brother wants you to be. At first glance, one might think that since Oceania is at war with Eastasia, it's natural that they'd want patriotic and devoted soldiers. However, as O'Brien tells him in then end, the Forever War between Eastasia and Oceania is of no importance at all, and merely one more way for the party to control the people. So in effect, this is what Big Brother encourages people to become.
The Seguleh form an entire nation of masked, battle-crazy warriors. That said nation only sent three people as their force against an entire empire is a testament to their prowess.
The warlike spirit nicknamed "Audrey" in "Okuyyuki" is a straight example. Unfortunately, she inhabits an ancient Japanese sword which has recently spent years cooped up in a dry antiques store. Thus, she is bored to tears, and very eager to be bought by an American soldier who might soon see combat when he comes by.
Oliver Twisted: The Knights of Nostradamus are good Magic Knights who take down evils and monsters, but Brownlow's particular eagerness to fight is apparent when he suddenly engages in a duel with Oliver not long after the latter picks up the weapon he was just given, just to see his capabilities.
Ryn: I enjoy breaking things ó breaking humans. It makes my spirit sing. And it is easy.
In John Milton's Paradise Lost, when the devils take council, Moloc is the first to speak and urges immediate and outright attack.
In Parasite Code by S. P. Tucker, Ava starts out as a delinquent teenager who relishes getting in fights with bigger, stronger opponents; getting the power to Megaton Punch people only makes this even more of a factor.
Sid from The Postmodern Adventures Of Kill Team One is very much one of these. He enthusiastically jumps straight to violence as a solution to even mundane problems. In Godless Murder Machine he disembowels a shoplifter at the Gamestop where he works. In Red Scare he severely beats a gun shop owner rather than fill out the paperwork necessary to buy a gun.
At the end of Godless Murder Machine he actually declares that killing people is way better than not killing people.
Talia in The Princess Series. When Snow first meets Danielle, she mentions how happy she is to have someone to talk to besides Talia. When Talia tells her to be quiet, Snow explains that Talia is only happy when she's stealing something or beating people to a pulp,
Inglis "Chris" Eucus the protagonist of Reborn to Master the Blade is eager to prove her fighting prowess and improve her sword skills as this was the whole reason she was reborn, her past life the Hero-King Inglis was a wise ruler but never really mastered the sword so this time she wants to become the very best swordswoman there is.
Heat has no problem accepting to his new demonic form and as a result, likes to rush headlong into battle to soothe his blood-thirst.
Serph absolutely relishes the thrill of the hunt, much to his horror. Subverted as he gets over his initial reservations but never becomes a full-blown Blood Knight.
Bat betrayed his leader Jinana because he thought she was too weak due to her pacifist methods. After that, he makes it no secret that he wants to eat everyone and becomes Heat's rival.
From the Reboot Book Series, Wren 178 is stated to enjoy "the hunt" — the hunt of humans, that is.
In Riesel Tales: Two Hunters, one of the two titular hunters, Runge Margavo, is more than happy to snatch up an assassination job and repaint the floor with someone's gray matter, and he's been described as someone who participates in bar fights with apparent regularity.
Katja in Eli Freysson's The Silent War is one of the setting's fated demon hunters, and so was quite literally born to fight. Her love of brawls meant every boy in the village feared her by the time she was twelve, and she is occasionally disturbed by the degree to which she enjoys bloodshed and danger.
Strong Belwas, a former pit fighter, delights in showing off his prowess to the point that he always allows his opponents to get one slash in before he kills them.
Sandor Clegane lives for little other than fighting and believes that "killing is the sweetest thing there is", and he's the good brother. The bad one, Gregor, just seems to like the murdering part rather than having a battle at all, and actually putting up a fight (which is bloody hard) seems to piss him off.
The Dothraki. They consider any event (like a wedding) with less than three deaths a dull affair.
The Smiling Knight was an Ax-Crazy Blood Knight who wanted to fight a member of the Kingsguard. He got his wish and met his end when he fought Arthur Dayne, the Sword of the Morning. Jaime witnessed the fight in his youth and considered it one of the best moments of his life; he also compares the Knight with Gregor Clegane above, musing that while he was only half as big, he was twice as insane. In a moment of self-reflection in the present, Jaime wonders how the boy who wanted to become the Sword of the Morning became the Smiling Knight instead.
Robert Baratheon, before he was King he was warrior who lived to fight, and when he took the Iron Throne he grew bored of managing the realm. He would have left the throne, and sailed to the Free Cities to be a sellsword. The only thing stopping him from doing it is his (bastard) son Joffrey taking the throne.
In Terry Mancour's The Spellmonger Series, almost every single one of the warmages from the first book are this, including the main character.
Mace Windu struggles with this in the novel Shatterpoint. You don't get to be second-in-command of the Jedi Order by going to town on everyone with a lightsaber, and Windu loves the stability the Republic provides, but he likes fighting anyway. He develops the dangerous lightsaber style known as Vaapad as a way to channel this aggression for good purposes.
The Yuuzhan Vong warriors live for combat, justified as they are raised in a caste system, and they live solely to fight and kill.
Jacen Solo AKA Darth Caedus is a lead from the front, get-stuck-in kind of fighter. It catches up with him in the end however, as the intensifying pace of battles leaves him increasingly debilitated.
Tash's first clone in Galaxy of Fear, is this as well as being Drunk on the Dark Side. So are the others, but she's specifically happy at the chance to kill Tash right then and there.
In the end, however, few cultures in the Expanded Universe fit the Blood Knight trope better than the Mandalorians. Battle is not only an activity all Mandalorians share in, it's one of the core components of their very culture. Though they have changed over millenia since the original race, the Tuang, brought other species into the culture, conflict has always been a part of who they are. This can mean anything from total war against the Republic, as it was in the early days; the struggle to survive against the environment itself; or personal battles and tests of skill and strength, such as the Great Hunts. Not all battles required ending in death, either, and in many ways the culture was still very honorable.
Jedi Master Gnost-Dural: Mandalorians believe confrontation is required for growth, on the personal as well as the cultural level. War is the Mandalorian way of life.
The Stormlight Archive: The Alethi people used to be a Proud Warrior Race charged with protecting the other nations from the Voidbringers, but when the Voidbringers were finally defeated, the Alethi never gave up their arms. Over four and a half thousand years later, the fighting is all they have left. The ideals of competition have pervaded every aspect of their society (even their religion has a points system), and they consider war the greatest of arts. Several times in the series, Alethi are asked "Why are we fighting," and the Alethi look at the asker like they're crazy. This is all made worse by the "Thrill," a nationwide bloodlust that afflicts the Alethi in battle, blunting the worst aspects of war even when they're on the front lines themselves.
In Oathbringer it is revealed that the Thrill is caused by a being named Nergaoul, one of the nine "Unmade" spren serving Odium, the setting's resident Big BadGod of Evil. More direct exposure to Nergaoul can corrupt armies into a berserk fury, even to the point of turning on their allies. Although some other the Unmade are sentient, Nergaoul itself is more akin to a force of nature than an intelligent being, acting as a pure expression of the need to fight and kill without malice.
Temeraire: The Hot-Blooded dragon Iskierka tries to fight an adult dragon literally as soon as she hatches and only gets more battle-eager from there. This ends up jeopardizing her position in the Aerial Corps when she repeatedly goes off to fight enemy dragons in defiance of orders, which once gets her and her captain captured.
Keeping in step with the trope's title, Radha of Keld in Magic: The Gathering'sTime Spiral card set and novels was intent on little other than fighting, at first spurning the world-saving efforts of Teferi and his companions... at least until he gave her the ability to kick even more ass than she could previously. She softens a bit as she grows to accept the necessity of helping Teferi, but she's always a hard-bitten warrior first and foremost. To wit:
"I hope to have such a death — lying in triumph upon the broken bodies of those who slew me."
Trapped on Draconica: All Leondians love fighting. This means their Prince, Kalak, will always choose fighting over any other solution.
Bad Tom of The Traitor Son Cycle got his nickname precisely because he's a combat monster who loves being in the thick of the fight. He even passes the sizable family fortune over to his younger brother because it'd keep him at home rather than out in the field with the Red Company.
Kage in Gav Thorpe's The Last Chancers. At the end of 13th Legion, he receives a pardon, resolves to stay in the forces outside the penal legion, and shortly thereafter loses it by brawling. In Kill Team, he is told that he used up his last chance and will never be pardoned again, and he can cope, realizing that he would have ended up fighting again.
Valerian in Watchers of the Throne becomes something of one after he gets the taste of real battle at Terra and Vorlese. When before he was content remaining a historian within the walls of the Palace, he now finds himself restless and yearns to use his Super Soldier abilities again.
Most notably, Tigerstar, Hawkfrost, Thistleclaw, and the other warriors of the Dark Forest all have a love for bloodshed and battle, which is even why Thistleclaw ended up there.
Blackstar counted when he was Blackfoot, being the top enforcer of Brokenstar, who was himself a brutal warrior who only used his Clan for battle purposes.
On the heroic side, Lionblaze counted when he was an apprentice, enjoying battle and scaring others with how brutal he could get.
Mudclaw supposedly being one was the reason Tallstar appointed Onewhisker as deputy just before his death, demoting Mudclaw and preventing him from becoming a bloodthirsty leader.
The Weakness of Beatrice the Level Cap Holy Swordswoman: The members of the guild Elkiad, who are former soldiers from the Cold War. They were dissatisfied at not being able to fight seriously on Earth (due to politics), so they took to the alternate world of Ground's Nir with glee. After becoming zombies, and hence unable to permanently die, this has only gotten worse.
The trollocs of The Wheel of Time are what happens when Blood Knight meets Dirty Coward. They're extremely bloodthirsty and love to fight, but really don't seem psychologically equipped to deal with defeat. So when they're winning, they're inhumanly aggressive and brutal, and when they're losing, their morale breaks almost instantly and they route easily. Whip-wielding Myrddraal ensure that no matter which mode they're in, trolloc forces remain pointed at the enemy.
Wild Cards. Billy "Carnifex" Ray is this, displaying increased strength, speed and healing abilities to get him back in the fight. This is a man who once pursued an opponent with half of his face cut off, and seems to revel in combat. Considering that he's a Wolverine Wannabe, it's not surprising.
Snowflake from Winter's Tale (Emrys Vaughn) claims to be literally addicted to violence, going into withdrawal if she goes more than a week without getting into a fight.
T. Sean Collins, the mercenary, in World War Z. He realizes that he is "addicted to murder" and will eventually either kill himself when he runs out of zombies to kill or face losing control and slaughtering actual people.