This is where the villain or a morally ambiguous character becomes obsessed with a character due to their witnessing said character go berserk and kill in a very messy or bloody way. Many times, there will be scenes where the villain will be somewhere watching the character fight violently and appear extremely turned on, saying something along the lines of "Marvellous..." If The Protagonist is no longer violent like that, often the villain will go to great lengths to make them revert back (these lengths normally involve kidnapping or killing their friends or love interests.)
If the villain / morally ambiguous character starts straight-out describing their euphoric emotions when they saw the good character massacre others, expect the good character to be verydisturbed and/or confused.
This can lead to the villain becoming a Stalker with a Crush. Compare Hemo Erotic and Monster Fangirl. May leads to lots of Foe Yay or No Yay.
Main antagonist Gauron of Full Metal Panic! seems to have a creepy attachment of this sort to the protagonist, Sousuke Sagara, who he first met as a child-soldier in some war-torn Middle Eastern country and saw something in that child's eyes - an ice-blue will to kill, without sin or care. Gauron spends much of the first season and the third one trying to 'reawaken' Sousuke's soldier spirit because he found it, in his own words, 'beautiful'.
Gauron is definitely the most extreme example in the series (getting aroused enough to have lamented about wanting to rape Sousuke after seeing the aftermath of all the people Sousuke calmly massacred). But Kurama, Gauron's partner in Amalgam, is shown to have started getting this way towards Sousuke as well. Although he initially shows annoyance and skepticism at Gauron's obsession and worship of Sousuke, when Kurama himself comes into confrontation with Sousuke, he starts to admire Sousuke more and more. After hearing that Sousuke massacred a bunch of his subordinates, he notes that "Mr. Iron was right. You are a very interesting boy."
Hunter ◊ Hunter: Hisoka towards Gon and Killua, calling them "unripe fruit" that he wants to pick when they become strong enough. But especially towards Gon.
Word of God says he's turned on by anyone with a strong aura.
It's especially obvious when he blushes during his fight with Gon in episode 5 of the 2011 anime remake and compliments Gon on how strong he is and how he loves his facial expressions.
Yuji towards Kitano after being defeated by him in Angel Densetsu. After some Character Development it evolves into real friendship, with Yuji declaring that Kitano is the best friend he's ever known.
Grimmjow from Bleach gets crazily excited whenever Ichigo proves to have gotten stronger during their battles together. While he initially thought of Ichigo as an annoyance that scarred his chest, he gradually grew more and more obsessed with defeating Ichigo. Some time after their second fight, Grimmjow's attitude towards Ichigo grew more intense, and he became determined to beat him.
This is a running theme in Bleach. Ichigo got this from Kenpachi in a previous arc.
Naruto: This is one of the reasons Orochimaru was so eager to obtain Sasuke's body. Later on, during his fight with Naruto, when Naruto is going into the third tail state, Orochimaru (who had until then had been going by Kabuto's assessment that Naruto was a talentless loser) smirks and says "How interesting..."
Upon seeing how much damage Kyuubi!Naruto can cause Orochimaru is shown smiling hugely and saying "This child...intrigues me to no end!"
Later in the second part we find Sasuke to have defeated large amounts of foes (without killing them, but still) and Orochimaru looks very pleased.
And possibly Kabuto. During a battle with Naruto in Part II, Naruto hits him with a massive amount of chakra that sends Kabuto crashing into the forest. Kabuto's response to this is to smile widely and think (almost in awe) "He hit me with pure chakra..."
Izumo no Okuni from Samurai Deeper Kyo is shown to get the trembles and be very aroused when thinking about Demon Eyes Kyo. She is even seen watching Kyo fight Jimon, saying outright that if Kyo doesn't revert back, she'll kill him and make a brochette out of him.
Kensei's aka Isshinsai Ogata's interest in Kenichi is a somewhat milder example. The first time he met Kenichi he immediately sensed his potential and offered to train Kenichi and make him into the strongest man in the world. When he observed Kenichi's battle with Odin he realized that Kenichi was already talented and wanted to claim him as his disciple even more. And he would have then and there if it wasn't for the Ryouzanpaku masters' Big Damn Heroes moment.
Not actually in love, but Jin'Eh from Rurouni Kenshin kidnaps Kaoru in order to try to get Kenshin to revert back to being the dangerous killer he was before, for the purpose of ensuring Kenshin would be fighting him with his full potential (as Kenshin is currently a Technical Pacifist, refusing to kill).
Legato Bluesummers from Trigun falls in love with Knives' violence and destruction, seeing him as a kind of messiah.
And, manga Legato also has Knives to look to for killing every single one of the people who had participated or been complicit in what appears to have been his repeated rape, leaving only little Legato alive. Of course, he didn't give a damn about the kid, but there's a reason for hero worship and sociopathology.
Another subversion in some of the Gung-Ho Guns Foe Yay with Vash — even though a Pacifist, they try to provoke him into the violence they're sure he's capable of, partly because of his bounty reputation and because they know about his relationship to Knives. Knives himself does this to get a reproduction of the destruction of July.
Diethard Ried from Code Geass is fascinated with Magnificent Bastards: he joins Black Knights after Lelouch purportedly blows up the leadership of fellow/rival resistance group to take control over the latter and later defects to Schneizel, believing that he is even crueler than Lulu. He is wrong and it doesn't end well for him.
Muraki from Descendants of Darkness has been shown to exhibit this somewhat. When Tsuzuki summons Suzaku and destroys everything, Muraki is shown to be very ecstatic (complete with the "Magnificent! Now I want you even more!" line).
Not a case of a berserker hero, but Graham Aker from Mobile Suit Gundam 00 becomes obsessed with the Exia Gundam and later its successor the 00 Gundam. He tends to ignore the other three (for each generation, no less) although all are equally worthy opponents.
Graham: "GUNDAM! I LOVE YOU!"
In the Blood+ manga, Solomon was like this for Saya when she goes berserk and kills. To put it in his words: "Wonderful... now that's our bride. Beautiful, but drenched in blood. It makes me want you, Saya." He later tells her that it was Love at First Sight because she looked so beautiful when she fought in Vietnam.
Carl to Saya to a slightly lesser extent in the anime.
Alucard from Hellsing has this with Integra, though it's as much her resolve as her potential for violence. Still, he goads her onward, and throws sexual references left and right when she finally orders the death of humans. Ironically, he is the Anti-Hero of the series.
He also implies that she feels this way toward him. She just tells him it's none of his business.
In Tenchi Muyo GXP, Knight of Cerebus Tarant Shank is awestruck by the sight of Kiriko covered in the blood of her enemies after she's forced to get dangerous. He continues to be interested in her when he reappears — for the little screentime he's left after the diminishing he goes through.
Mukuro from Katekyo Hitman Reborn!, in regards to Tsuna. For example, in chapter 385 he's shown looking quite pleased when he sees Tsuna defeat a foe and remarks that "He's gotten even stronger."
Only when Tsuna goes into dying will mode and defeats Gokudera does Gokudera drops his accusations of Tsuna being a weak loser and take to the idea of Tsuna being the tenth Vongola boss...and he becomes quite determined to be Tsuna's right hand man...
Squalo's undying devotion for Xanxus is later explained by Squalo as due to his being fascinated by Xanxus' "rage" (which he saw initially in his eyes). Of course, it becomes increasingly obvious that this is probably because Squalo is a masochist that likes seeing Xanxus go berserk and beat people (including Squalo himself) up.
Blood KnightEnsuu is thought to be this. Her fixation on getting to fight Ayana again stems from her opinion of the other girl as someone who is of the "same kind" as her.
In Shut Hell, Alfald is this towards the eponymous heroine. He's shown to get crazy excited when he sees her killing, and becomes aroused when she kisses him after ripping a man's throat out with her teeth.
Alfald: "Yes, show me even more, Shut Hell. More than gold, more than women, fascinate me more. With this wind of violence only you can show me. Take away my soul!"
In Noir, Chloe becomes fascinated with Kirika after witnessing the latter's bloody efficiency in slaughtering Mireille's entire family.
Tsukuyomi can get like this when fighting Setsuna. In chapter 293, Tsukuyomi has a typically weird and disturbing moment when she gets all gooey over Negi's new level in badass, and it resembles nothing so much as somebody catching themselves fantasizing about a friend's significant other.
Fate also does this sort of thing when he fights Negi, leading to some Foe Yay.
Fate: "Beautiful. That's what I'm looking for, Negi-kun."
Haguro is in love with Inugami's carnage, to the point he refuses to believe Inugami even has the ability to like normal people.
In Kara no Kyoukai, this is the reason that Lio Shirazumi is after Ryougi Shiki (the main character) in the 7th movie. His goal is to turn her into a complete homicidal maniac like him so that they can be together. Unfortunately she's taken, so he decides to "kill what's holding her back" to get her to "come around". He only succeeds in getting Shiki to utterly massacre the bastard when she thinks that Mikiya is dead.
Liang Qi of Canaan is like this for Alphard. She appears calm and even-tempered, but is in fact unwavering, merciless, and very much obsessed with admiring and idolizing the absolutely Badass Alphard to the point of being in love with her. To her frustration, however, Alphard does not reciprocate her rather twisted feelings and instead ignores or even proceeds to ridicule her from time to time. Because Alphard seems to have taken a special interest in observing Canaan's reactions to every plot carried out by Snake, Liang Qi sees the elimination of Canaan as a way to win back Alphard's attention.
Lua seems positively ecstatic when Ladd is giving his speech about murdering the other train passengers in the third episode. Not that Ladd is a paragon of virtue by any means, but it's that kind of series.
When Ladd saw bodies massacred by Rail Tracer, he was dancing like a 5-year-old child in a puddle of blood, laughing maniacally and screaming "I CANNOT WAIT TO MEET THE GENIUS WHO'S RESPONSIBLE FOR THIS MASTERPIECE!"
In the manga version of Chrono Crusade, Aion gets a huge grin on his face when he witnesses the carnage left behind after Chrono's Berserk Button is pushed. He later compliments Chrono heavily for the fight, and when Chrono starts trying to use more self-control in his battles he strongly expresses disappointment in Chrono "weakening."
In Get Backers after Ban ripped his arm off, Fudou became totally obsessed with him. He actually kept his own rotting, maggot-infested arm with him just so he can remind himself of Ban - which made him feel "incredible chills rising up throughout his body."
Berserk: Griffith toward Guts. He states that Guts' fighting is "magnificent" and for that reason wants him to join his Band of the Hawk. Their relationship becomes much more possessive as time goes by.
Revy from Black Lagoon is quite appealed by Genji even before he draws his sword. When he pulls off an actual bullet slice and proceeds to literally disarm Chaka, she begs him to fight her. In a tone that demonstrates pretty obvious arousal...for Revy, anyway.
Inverted in Jhonen Vasquez's Johnny the Homicidal Maniac. Bystander Jimmy develops an infatuation with our villain Johnny when he witnesses Johnny's killing spree at a Taco Hell Outlet. He goes out of his way to imitate Johnny, and even tracks Johnny down to his house.
And then Johnny kicks his ass and guts him like a trout for being such a creepy fanboy and obsessing over such depraved acts.
That and because Jimmy raped his victims, which was what drove Nny to make him suffer a bit before he finished him off
In Hack Slash Vs. Evil Ernie, the Ax-Crazy title crossover character falls in love with Cassie after telepathically seeing how horrifically violent her life is and how screwed up she is, causing her no small amount of trauma and moral conflict.
Murder Messiah features a Serial Killer who has a similar obsession with Cassie, though he admits any attraction he feels towards her is secondary to his primary goal of figuring out how to become a slasher.
The 2000 AD comic Button Man is about rich folks paying guys to kill each other. The main character's 'sponsor' demonstrates distinct, ah, excitement over the violence and bloodshed, although it doesn't appear to be sexual in nature.
Judge Dredd: Judge Death in his Origins Issue reveals that during an investigation when he was still a human Judge, he met the Sisters of Death, two occultists who worshipped death and practiced The Dark Arts. He let them live because he was quite intrigued by their carnal activities, already being quite the murderous creep himself. He participated in their rituals, where it's implied that they regularly tortured, murdered and ate people and held orgies. They later used their magic to turn him into an undead spirit so he could truly be in a position to judge everyone for committing the "crime" of living.
Miek of the Warbound was in love with The Hulk's carnage. He allowed Sakaar to be destroyed in order to keep the Hulk from becoming a peaceful ruler instead of the World Breaker Miek worshipped.
Bullseye in the eponymous arc of Punisher MAX. See his internal monologue as he's watching Punisher slaughter his men: "You do not kill like any man I've ever seen, Frank. You're more like a force of nature. An earthquake or a tidal wave. A tornado. Watching you kill is like watching Rembrandt paint or hearing Mozart conduct his 9th symphony. You are the most beautiful thing I have ever seen, Frank Castle. I think I'm going to cry. Thank you, Frank. Thank you for being you." Then later when talking to Kingpin about the encounter: "I saw him in action. Let me tell you, it was... it was something to behold. I do apologize, but it appears I'm going to need to pleasure myself now."
In one issue of The Punisher, the Punisher teams up with Elektra. At the end of the issue, after their adventure, Punisher has an internal monologue where he muses about just how violent, unstable, and overall crazy Elektra is. He then asks her out on a date (although this happens in the final frame, so we never know if she said yes or not).
The comic "Red Romance" tells the tale of two murderers that fall in love with each other's brutality. When their sadomasochistic love life and their collaborative team killing grows boring, they try to rekindle the fire with some help. Things get really romantic and messed up.
Red Lantern leader Atrocitus falls in love with Mera's carnage (or maybe just Mera period) when she is turned into a deputy Red Lantern during Blackest Night. The first energy construct he creates, as opposed to just vomiting red plasma everywhere, is Mera. Star Sapphire Carol Ferris even notes that there is a spark of love in Atrocitus' heart. Not that Atrocitus will ever admit it.
Shriek fell in...love?...with the Spider-Man villain actually named Carnage when she saw him massacring his way through the insane asylum she was imprisoned in, in the opening chapter of Maximum Carnage. "Make mine mayhem!" He broke her out and they "adopted" several other villains as a perversion of every good family ideal that seriously creeped out even several of the superheroes opposing them.
In the Archie Sonic the Hedgehog comics, the trigger happy, robotic, antihero, E-123 Omega, develops an amusing crush on Blaze the Cat after she defeats him and the rest of Team Dark with her Pyrokinesis. This leads to an amusing scene where he says "I LIKE YOU. LET US BURN THINGS TOGETHER." To a surprised Blaze while holding her hands in a romantic way, and later, bashfully tells her that she burns things very well when they part ways at the end of the story arc. In the non-canon character Q&A on Ian Flynn's message board, he also asks heroic AI Nicole to literally have his robotic kids after learning of the potential carnage she could create if she went all out.
Harmony Theory: After Charisma sees what Rainbow can do, she seems impressed and starts calling her "Hot Stuff". She is also pleased by Blaze's resistance to torture.
Suž-Fēng in To Prey Upon the Dead is highly disturbed to realize Unohana is getting off as Suž-Fēng describes how she was defeated.
Later Unohana comes across an area Ichigo and Kenpachi were fighting in and barely restrains herself from charging off to fight Ichigo herself.
Ichi the Killer has a poster boy for this: Masao Kakihara. He is actually shown getting stiffies when he sees the over-the-top carnage Ichi creates. Being a masochist, he stalks Ichi in the hopes that Ichi will torture him.
Palpatine in Star Wars cackles, grins, and eggs on Luke Skywalker as he unleashes a rage-fueled No-Holds-Barred Beatdown on Darth Vader, both because he enjoys watching it and because he wants to turn Luke to The Dark Side. He also does this with every other fight we witness where one 'good' character is going berserk. Some (not all) other 'evil' (not necessarily Sith) characters also do that, especially in the expanded universe. (Dark Forces series antagonists for example... they love when the player goes into a murderous rampage. A particularly interesting example is the game Jedi Academy, where the antagonist encourages the player to kill an annoying character that was supposed to be a friend and is a traitor, and players are likely to do it, just because that character IS ANNOYING.)
Kingdom of Heaven: As the protagonist use more and more inventive ways to massacre the antagonist army (like attaching ropes and then pulling sideways the towers designed to climb walls), the antagonist (Saladin) is clearly seen smiling, the more his troops get massacred, the more he smiles, like if he was thinking along the lines: "Cool! I should use this in my next battle..."
Sebastian Shaw bursts into laughter and shouts stuff like "Wonderful!" and "Excellent!" when Erik Lehnsherr as a boy slaughters all of Shaw's men and trashes his office. Later, they meet when Erik is an adult, and Shaw honestly compliments him on how powerful he has become, even calling him his son.
It's a blink-and-you'll-miss-it moment, but Charles Xavier's face glows with delight when he witnesses Erik using the anchor of Shaw's boat as a destructive tool.
In Animorphs, The Drode is rather fond of Rachel's rage and bloodlust.
In the Anita Blake series, the serial killer Olaf develops the hots for Anita after seeing how efficiently and ruthlessly she kills vampires.
In Skulduggery Pleasant, when a Remnant sees a vision of the future, where Darquesse/Valkyrie is reaping destruction, this inspires it to break out and spread the word among other Remnants who now worship Darquesse as their saviour. This leads to many of them more-or-less reciting the creepy line, "We love you Darquesse."
This takes a pretty creepy turn when they try to manifest Darquesse by attaching a Remnant to Valkyrie and the results cause them to panic... all except for that first one, who goes so far as to cripple the ones who try running away.
Horace Harkness of the Royal Manticoran Navy was pathologically incapable of walking by a Marine without starting a brawlnote the Marines took it as a compliment, as the two services have a friendly rivalry. Then he met Sergeant Major Iris Babcock and found himself absolutely dumbstruck by her grace and lethality in a fight. The next time either of the characters appear, they're married — and Harkness is no longer brawling with Marines.
Ares was madly in love with Xena when she was a warlord, and most of his villainous schemes against her were in hopes of getting her to revert to her old ways.
"...and if you start my heart, it's just the way you slash and burn."
Callisto does her fair share of this too, although some of it's probably tied to her half-hearted death wish.
Apparently several vengeance demons were very impressed by Willow's Heroic BSOD after Tara's death to the point that their leader D'Hoffryn has a standing offer that Willow can join them whenever she wants. He also mentions that one of the VD's has a framed picture of her flaying Warren alive, considering it a work of art.
D'Hoffryn already had his eyes on Willow when she accidentally cursed her friends in "Something Blue".
In Doctor Who, the Master, having missed the end of the Time War, is intrigued to learn that the Doctor has destroyed both Gallifrey and Skaro.
In "Asylum of the Daleks", it's revealed that Daleks are occasionally forced to lock up Daleks who prove to be so dangerous, even they can't control them! Since they are obsessed with purity, the Doctor asks why they don't simply exterminate them? The Dalek Prime Minister explains that they can't bring themselves to remove such pure hatred from the universe. It is beautiful to them!
The Dalek Prime Minster suggests that perhaps that's the reason why they've never managed to kill the Doctor.
An episode of Criminal Minds had a copycat killer suffering from the below mentioned hybristophilia.
Burn Notice has Larry feeling this way about the Michael Westen "he knew".
In True Blood, the vampire Godric observed the Viking warrior Eric Northman's prowess in battle, and was so impressed that he turned Eric into a vampire so that he could fight forever.
Godric: I watched you on the battlefield last night. I never saw anyone fight like you.
Eric: I would fight you now if I could.
Godric: I know. It's beautiful.
In the second season of NBC's Hannibal, Hannibal Lecter takes a very personal interest in Will Graham's Start of Darkness. The two of them have intimate dinners together to plan and celebrate murders.
Ocelot from Metal Gear Solid 3: Snake Eater expresses pseudo-romantic admiration for Big Boss when he hears that Big Boss so easily defeated the members of the Cobra. Volgin notices, asking Ocelot if he's "fallen for him" because of this. Not to mention how he saw Big Boss in a Gaussian Girl manner after Big Boss massacred his entire Ocelot unit.
In Fallout 3 Mr. Burke will make comments such as "natural selection at its finest!" if you stealthily kill someone in his presence. Though attacking him causes crybaby screams of how you're a masochist.
Nobunaga: "Noh. You're beautiful when you're slaughtering peons."
Noh: "Shall we reap eternal suffering my love?"
Grimnaw from Neverwinter Nights. He isn't a villain; he's an ally, but his main reason for connecting with your PC is all the death your PC causes...
Tira from the Soul Series has an infatuation with chaos and destruction, so when she met Nightmare, it was unrequited love at first sight. However, in Soulcalibur V, the new Nightmare is a political entity who absorbs souls in secret. Tira was not pleased to discover this and started seeking out a new host for Soul Edge.
HK-47 in Knights of the Old Republic 1 and 2. He's pretty much evil already, but his regard for your character goes up or down depending on how many cruel acts you perform in his presence.
HK-47: Statement: Master, I must say it is a pleasure working side by side with you.
Exile: If you have a long-winded explanation for why, indulge me.
HK-47: Statement: Just when I believe my photoreceptors have recorded the last potential aspect of your cruelty to my memory core, you commit a new atrocity that leaves me analyzing its impact for days.
HK-47: You are like a delightful random cruelty generator, master, poisoning all you touch with your presence. You are a testament to all organic meatbags everywhere.
Exile: Stick with me - you'll pick up a few things.
HK-47: Statement: I have already learned a great deal, master, and I am anxious to learn more about lying, betrayal, and new ways to harm innocents.
Darth Malak has shades of this towards the protagonist, who's the amnesiac Darth Revan and his former Master. During the climatic battle on the Star Forge, he turns his entire forces towards them, fully admitting that he expects all this will achieve is to slow them down temporarily.
In Manhunt, Lionel Starkweather apparently gets very turned on when the player has protagonist James Earl Cash kill people in violent ways. In some dialogue, it is implied that he is possibly masturbating to Cash's murder spree. He expresses disappointment in the character if the player chooses to not kill people.
In The Suffering, Hermes, Blackmore, Copperfield, and the Creeper all take their turns in praising Torque for committing violent acts. Indeed, one of the final levels of the second game is devoted to a review of Torque's decisions during the game, conducted by three of the Ghosts. Ironically, should you take the evil path all the way, the Creeper stops complimenting you when he realises that you're a danger to his reputation; a Boss Battle follows.
In Sacrifice, Charnel will offer boons to Eldred during the campaign for acts of particularly wanton cruelty... Even if you're not currently working for him. In one case he'll even reward you for killing one of his own minions, because the person was only giving him lip service anyway and seeing you kill him was so much more entertaining.
Ares in God of War was impressed by the brutal Spartan warrior that Kratos was, and gave him the Blades of Chaos to save and inspire him to even more spectacular acts of carnage. He got his wish, although Kratos didn't appreciate Ares arranging the death of his family and thus removing his only "weakness".
In Mass Effect 2, if Grunt's in your party when you're chasing down Jack, a biotic powerhouse with serious rage issues, during her recruitment mission, he'll spend much of the level admiring her appetite for destruction.
During Grunt's loyalty mission on Tuchanka, if Grunt and Shepard manage to take down the Thresher Maw, EDI reports that the Krogan have sent several breeding requests for him... and one for Shepard.
Harbinger's obsession with capturing Shepard could be interpreted as stemming from this.
In Mass Effect 3, if Grunt wasn't recruited or accepted into Clan Urdnot, he is replaced as leader of Aralakh Company by Urdnot Dagg, who expresses envy and admiration towards Shepard for having proven such a foe that every Reaper in the galaxy wants to kill them.
A Shepard romancing Kaidan outright states that she is turned on by watching him fight in the Citadel DLC (a rare heroic example, unless she's a Renegade).
By Mass Effect 3, it's shown that krogan as a whole, hold Shepard in quite high regard, especially for someone who is not a krogan. Even Wreav, who considers krogan the apex species, treats Shepard with a grudging and reluctant respect. A good reason for this is because Shepard is shown to be feared by the near incomprehensibly powerful Reapers.
Though not a romantic example, as far as we know, Kefka has this attitude towards Terra in Dissidia: Final Fantasy. He particularly seems to enjoy telling her, after she recovers from going berserk, how she beat up an ally and how much she seemed to enjoy it. In his prefight quote, he also shows a similar attitude towards Shantotto.
"Now, you've got what it takes to destroy the world!"
This motive was hinted at in the original game and is similar to Sephiroth's assertion that he controls Cloud's fate. Really, it's present with several Dissidia characters. The Emperor tries to seduce Warrior of Light and Firion to his side, remarking that they're "intriguing" or "pawns better off surrendering to me."
The Emperor is even implied to have that same view towards Terra, as seen in one of his pre-liners if they face one another in a fight, though the specific line can also have other connotations.
Kotohime (victory quote to Mima): Oh, you beautiful barrage! What a dream!
Shiki of Togainu no Chi gets quite excited whenever Akira puts up a fight and appears to get stronger.
In Asura's Wrath, Sergei's reaction when Wrath Asura impales him right through his midsection with his handreeks of this. He thinks that Asura's wrath is "glorious".
In Nier, Tyrann enjoys killing Shades very... thoroughly. And due to his possession of Kaine, so does their body, as revealed in Grimoire Nier.
Team Fortress 2: All of the classes (as well as the Administrator) tend to be quite sadistically gleeful when things go well.
In Baldur's Gate II, an evil dark elf cleric named Viconia DeVir can be romanced by male players. And she will be attracted to the fact that the player is a child of the God of Murder and is capable of great destruction.
Final Fantasy XIII: Orphan sounds just a little too excited at the prospect of Fang transforming into Ragnarok and destroying him, yelling its praises and encouragement as she gives into anger and attacks - then blasting her with magic and revive spells when she fails, to try and make her do it again. Barthandelus is slightly less subtle, yelling "RELEASE" in a booming voice as the party kill him.
In Narbonic, Sociopathic Hero Mell Kelly exasperatedly asks Caliban the ex-demon why she still dates him. His response is to gaze deep into her eyes and tell her in a seductive tone it's because when she looks into his eyes she sees "the hosts of the damned, writhing and screaming in the forges of pain I stoked for untold centuries". Oh yeah◊...
Caliban: All that time I spent as a goalkeeper of souls, it'd be a shame if I couldn't hold onto one.
Bort the mongrelman from Dominic Deegan swoons over the mongrelwoman as she slaughters a group of men who threw a net on her in "Escape from the Wild Edge". Odd in that Bort isn't the villain of the story.
In Marilith, Big Bad Valentino first becomes interested in the Hitman with a Heart Marilith after hearing about a particularly bloody job the latter performed. In fact, the body count was only so high because the job went wrong, and Marilith is nowhere near as Ax-Crazy as Valentino. Valentino takes this revelation... poorly.
Homestuck: Kanaya has just come back from the dead, kicked an Ax-Crazy psychopath off a cliff, decked Vriska so hard one of her pupils falls out of place, and chainsawed her murderer in half. Vriska's response?
An odd, mutual example occurs in Survival of the Fittest where Clio Gabriella and Maxwell Lombardi (two of the top killers) meet each other, and... start to become really fond of one another. It's implied that Clio felt like Maxwell was one of the few people to understand her actions, but it still has undertones of this trope on both sides.
In Hellsing Ultimate Abridged the Queen asks Alucard, her former lover, to record it when he kills the Major so she can watch the video every night when she goes to sleep. Alucard is thrilled.
An episode of Super Power Beat Down pits Nathan Drake against Lara Croft. The two have a gunfight while trying to take possession of a priceless artifact. Ultimately, Lara Croft gets the artifact. As she makes her escape, Nathan Drake watches her go and mutters "I think I'm in love."
In Teen Titans, when Slade has Robin as his apprentice, he watches Robin beat up the rest of the Titans and says that it's "thrilling." He even rewinds the tape a couple times. Make of that what you will.
Danny Phantom: In a similar vein, Vlad enjoys taunting Danny about their supposed similarities (about how he's becoming more like Vlad with every battle) and he appears extremely satisfied and entertained whenever fighting Danny.
Drawn Together pretty much shines a light on this trope in "Spelling Applebee's", where Princess Clara uses Captain Hero to create car crashes so she can get off on the resulting carnage. Subverted somewhat when Hero causes a crash which kills Clara's best friends.
Family Guy: After Lois does some fighting moves, we cut to Quagmire naked in his house and watching from his open window stating "That was strangely arousing." The window then falls on his penis and gets stuck. He calls 911 and reports "Yeah, its in a window this time."
The Simpsons: In a parody story of Bonnie and Clyde, Homer (Clyde) thinks to himself, "Violence must be her turn-on! Well, there are probably others, but I'm too lazy to try and figure it out..."