Claymore might have the most straightforward interpretation of this trope, as the Claymore warriors feel greater pleasure the further they utilize their demonic (yoma) powers. In fact, part of the reason why all Claymores are female is due to how male warriors are unable to suppress the need to transform all the way once they start using their yoma power, a less than subtle allusion to ejaculatory inevitability (in fact, it's downright explicitly stated that releasing yoma power is akin to the feeling of orgasm).
In the Battle City arc of Yu-Gi-Oh!, tombkeeper Marik Ishtar has another, psychotic evil personality (who is actually NOT another soul unlike with Yugi and Bakura) that is sealed by the consciousness of his half-brother Odion (Rishid in the original). When Odion is knocked unconscious by The Winged Dragon of Ra, the evil Marik comes out. He remarks about how weak his other side is, and how scared he is of the dark. "But not me, I love the dark. It fills me with purpose!" Yami Marik gloats. He then proceeds to show how insane he is by rendering Mai and Joey (Jonouchi) comatose after putting them through hideous torturous Dark Games.
From the example above, Vegeta of the Dragon Ball Z fame allowed himself to become taken over by the evil wizard Babidi, in his Batman-Gambit to be strong enough to defeat Goku, making this More than Mind Control as opposed to Brainwashed and Crazy. He clearly could have had the will to resist Babidi's attempts, and later proved himself able to do so when he tried to give out his commands. However, Vegeta was merely interested in fighting Goku. He explained to Goku that he felt that his badassery was decaying after settling down, and forming a family, so witnessing the power of Babidi's magic, he became possessed on purpose... and was very pleased with the power boost.
In the dub of YuYu Hakusho, the younger Toguro brother (a human turned into a demon for winning the Dark Tournament a long time ago) mentions that, while demonic energy is less powerful than human spirit energy, it "gives a much more pleasurable sensation when used."
One of the grittier aspects of Ranma ½ is the very well-established presence of this trope. Ruthlessly amoral and self-centered characters like Nabiki Tendo and Happosai are almost always serene and upbeat, usually getting down only when things don't go their way (which doesn't happen to Nabiki). More moral and decent characters, such as Ranma Saotome & Ryoga Hibiki, on the other hand, tend to often be depressed as their conscience punishes him for the various ill-deeds they either perform or allow to happen. The best example are the Polar Opposite Twins Pink & Link; Pink is a sadist who gleefully poisons anyone she sees just for laughs, while Link is a noble altruist who considers it her duty to heal anyone her sister poisons — and because they are identical twins, is often beaten up by her charges, as they can't tell the difference. The easiest way to tell them apart? Pink is always smiling, Link is always frowning.
Sasuke of Naruto ranting about how killing Danzo felt great.
Slan of Berserk comments during the Eclipse on how "delicious" it is when Griffith as Femto rapes Casca to insanity in front of Guts who's forced to watch it happen. Gets even creepier when it sounds like she's having an orgasm.
This trope was a staple of the Nineties Anti-Hero genre (as well as their predecessors, notably Wolverine).
Miss Misery of the comic book Sleeper was a villain who suffered from an interesting take on this - not only did being evil make her happier, healthier, more beautiful, and physically stronger, but being kind and unselfish was hazardous to her health. Back before she became a criminal, she was nice, ordinary, and extremely ill. Becoming amoral and hurting people elevated her health to the point of perfection. Once she fell in love with the main character, she started falling ill again.
When Red She-Hulk's identity is revealed to be Betty Ross Banner, she tells Bruce Banner how she became what she is.
Red She-Hulk: "They filled me with rage... stripped away my will... but you know what? I kind of liked it. You understand what I'm talking about, Bruce. Maybe you're the only one who really can. That insane rush of really cutting loose, of not caring at all what will happen as a result... just smashing and smashing and smashing anyone they sent me against."
Rex the Wonderdog gets a taste of this when he puts on a hellhound's collar and transforms into one to spy on the forces of Hell. He claims that it's the first time he ever understood the desire to be a "bad dog".
In Sin City, most villains mention how good it feels to be evil. The Yellow Bastard referred to it as "having the time of [his] life." Eva Lord seemed to be a little too excited by the prospect and Senator Roarke loved the gloat that he could get away with anything he chose to get away with. While the Serial Killer Kevin never spoke on panel, he had mentioned to his surrogate father that eating people "filled him with the touch of God", although he also felt guilty for his actions. Finally, even the anti-heroes of the universe get in on this, often finding a great deal of joy when killing bad guys.
In the 1940s, a German boy named Johann Schimdt tried to woo a Jewish girl. When she turned him down, he killed her... and realized that he enjoyed the feeling. He joined the Nazi Party in order to get more chances to feel that rush, and then one day happened to meet Adolf Hitler. This is where the Red Skull got his start.
Forever Evil: Johnny Quick and Atomica love doing evil things for the fun of it.
Ginger Snaps: "[Transforming into a Werewolf]- It's like touching yourself. You know every move. Right on the fucking dot. And after - see fucking fireworks ... supernovas. I'm a god damn force of nature. I feel like I could do ... just about anything."
Star Wars: The Dark Side is the easier side of the Force from which you can draw power, making it tempting to use it as a shortcut to awesome. It is also the "more seductive" side, as Yoda puts it, giving it an addictive quality. Once you've used it once, the temptation to use it again becomes harder to overcome, resulting in a vicious cycle of addiction that just can never end well.
When Dr. Channard becomes a Cenobite in Hellbound: Hellraiser II, which comes with a fair amount of body horror, and a giant tendril burrowed into his head, what is his reaction?
Channard: And to think... I hesitated.
It's stated in the fifth and seventh Harry Potter books that one has to "mean it" when using an Unforgiveable Curse for it to work properly.
Harry ends up using two of the Unforgivable Curses to great effect in the seventh book. One of them was used at a moment of high emotion, however (when McGonagall is insulted, although this could be Disproportionate Retribution), and the other is more out of necessity when infiltrating Gringotts, when the alternative would be murdering the guards and all the witnesses.
This starts to happen to the main character of the Goosebumps book The Haunted Mask.
Referenced in The Wheel of Time, in Elyas' advice to Perrin concerning his axe: specifically, that he should keep it until chopping things up starts to feel good, at which point he should throw it as hard as he can and run the opposite direction.
Which he then, some ten books later, actually does (for good reason).
The overt descriptions of the ecstasy that the Chosen/Forsaken are stated to feel whilst basking in the presence of Dark One. Graendal further elaborates, implying that while some of the Forsaken were always morally and ethically challenged, others had to really apply this trope to themselves in order to eradicate their values—evidently, it worked.
Also an aspect of the Discworld character Vimes, who regularly has to fight back his "inner beast" during the later books as to avoid slaughtering particularly despicable suspects/criminals. However, there are occasions when he decides to unleash it to great effect.
The Strange Case of Dr. Jekyll and Mr. Hyde is a trope codifier, probably (not most of the adaptations). The reason Hyde keeps using the chemical that brings his "evil" side out is because he feels he is, then, allowed to do as he pleases, though he does try to stop when his conscience catches up to him.
Fire in the Blood has Vampire P.I. Jack Fleming realize this when he hypnotizes Doreen Grey and nearly drains her dry. He spends much of the rest of the book resisting the urge to do it again.
In The Picture of Dorian Gray, Dorian Gray starts as an innocent and beautiful boy worried about his beauty going away with his youth, but after the suicide of Sibyl Vane and the change in his picture, he notices that he doesn't feel anything about it (and the picture absorbs most consequences), leading to him quickly becoming addicted to various vices.
Defied in The Screwtape Letters, where evil is not at all fun, and good demonic style means gaining a human soul while offering nothing in return.
In AllTheRage, when Jack is unwittingly dosed with Berzerk, his normally-stable inner monologue gives way to power fantasies in which he feels like he's the rightful owner of the city, the road, and everything around him. And too bad for anyone he kicks the crap out of for getting in his way, because they were dumb not to have known better.
In the Transformers: TransTech story "I, Lowtech", when executive Bulletbike loses his "clarity codecs", he discovers his first accidental evil/violent act... actually wasn't that bad, really. And then keeps going from there. At first he pretends he doesn't enjoy it and justifies it as being for nobler purposes, but eventually progresses to reveling in it as his mental justifications become more and more half-hearted, petty, and selfish.
In The Year of Rogue Dragons, The Rage actually feels good when dragons stop resisting it. Karas resisting it so much while she seems to crave for it speaks gallons about how heroic she is.
In Smallville, when Clark Kent is affected by red kryptonite, his "Kal" persona surfaces, and he has mentioned that being ruthless and uncaring feels good.
The eponymous character of Angel has commented that he misses the "clarity" of having no morals. Angelus does seem to truly, deeply enjoy being infamously evil.
Buffy plays with this a lot really, mainly using Buffy and Willow.
Mentioned in season 6 when Spike encourages Buffy to do things his way, which will include all sorts of badness and mixing with evil creatures. "Try on my world. See how good it feels."
Comically subverted immediately afterward when Spike's big plans for the night involve gambling kittens with a group of Ambiguously Evil demons. Buffy sits in the corner drinking and snarking at Spike about it.
Faith: "When are you gonna get this, B? Life for a Slayer is very simple: Want. Take. Have." She later interprets her own advice as her justification for trying to rape Xander, becoming This Is Your Brain on Evil.
Buffy runs into a former schoolmate who's been turned into a vampire.
Feels great. Strong. Like I'm connected to a powerful all-consuming evil that's gonna suck the world into a fiery oblivion. How 'bout you?
George (a werewolf) has a great speech about this in Being Human.
Walter White of Breaking Bad alternates between Evil Feels Good and Being Evil Sucks. He finds he likes the power and feeling being a drug lord brings.
In the Grand Finale, he finally stops lying to his wife and himself and admits that everything he did, he did for himself because he liked it and was good at it.
Morgana from Merlin looks positively orgasmic every time she kills an innocent person, plots against her friends and family, or performs destructive magic.
Surprisingly and heartbreakingly subverted when she actually does succeed in killing Uther. Morgana tells Agravaine she isn't celebrating her victory because Arthur is still in the way but she is obviously shocked and deeply upset - and baffled at her own total lack of joy.
Once Upon a Time: After using magic for the first time, Regina (The Evil Queen) tells Rumplestiltskin that she'll never use it again... because she loved it. Magic itself isn't inherently evil in this universe, but since in this case it's explicitly being talked about in a bad way, it certainly qualifies here.
In S&M, Rihanna reminds us that "Feels so good being bad / There's no way I'm turning back".
As a general rule, this trope frequently occurs in the lyrics of Horrorcore artists.
In Calvin and Hobbes, Hobbes stops Calvin from smacking Susie in the head with a snowball, and counsels him that true happiness comes from a life of virtue. Calvin spends the day being a model son, but upon reflection finds himself deeply unsatisfied, so he completes his original task of pasting Susie with that snowball, then laughs his head off. "Virtue needs more cheap thrills," observes Hobbes.
In the Captain Kremmen radio serial by Kenny Everett, our hero demands his Evil Twin give up his evil ways and surrender, which leads to yet another No Fourth Wall gag.
Evil Kremmen: Never! I'm enjoying being a baddie! It's much more fun being a baddie! Isn't that right, kids?
Even The Bible admits the validity of this trope in the book of Hebrews, when it says that Moses forsook being called the son of Pharaoh's daughter, choosing to suffer affliction with the people of God, than to enjoy the pleasures of sin for a season.
This is one justification for belief in an afterlife: giving up pleasure in this world in order to have eternal pleasure in the next.
Many of The World of Darkness games explore this concept through their Karma Meter system, Morality (moreso in the nWOD than the oWOD). It's explicitly stated that maintaining a high Morality/Humanity/etc. is done by feeling remorse and horror at one's misdeeds. What causes it to degrade is reveling in one's power and enjoying the feeling.
In the world of Mage: The Awakening, lower Wisdom can give the character access to greater powers at the cost of his sanity.
In Vampire: The Requiem, a Vampire who loses too much of his Humanity becomes less able to avoid flying into a Frenzy of rage or terror. A big element of the game is balancing one's nature as a predator against maintaining control of one's humanity.note Humanity also affects other things-your Humanity is actually the physical cap on the number of dice you can roll during daylight hours or while interacting socially with humans, because the lower your Humanity the deeper you slide into the Uncanny Valley.
Averted with Demon The Fallen - sure, being evil kinda feels good at first, but their Karma Meter is called "Torment". The more you give in, the more you're haunted by your memories of Hell, and the more you lash out at those around you because of it. Hit Torment 10 (it goes up, not down like most meters), and you are completely consumed by your hate, unable to do anything but rage against the world around you.
Chaos in general for both Warhammer and Warhammer 40,000 is this trope played ridiculously straight and horrifically subverted, acquiring a near-constant supply of recruits despite being Always Chaotic Evil due to the overwhelming, almost cathartic release it provides, but over time their actions and the fickle nature of Chaos itself dulls any sense of satisfaction until they must reach and exceed increasingly brutal extremes to feel anything at all.
Played especially straight with the followers of the Chaos God Slaanesh, also known as the "Prince of Excess," or "Prince of Pleasure." To say they are Too Kinky to Torture is akin to saying that... well, it usually makes them stronger.
The Dark Eldar are a society of Always Chaotic Evil space elves who will have their souls drained away if they don't torture people. In one story, one high-ranking member of the species tries to woo another. He sacrifices everything, including his most trusted servant/best friend/sorta daughter, and only then does his beloved accept him, because he now has "A hole in his heart," that will stay there forever. It was semi-obvious that she was having an evilgasm as she said this.
Blackguards in Dungeons & Dragons (pre-fourth edition) actually get BONUS abilities for being fallen Paladins.
The newspaper comic strip Tumbleweeds was once adapted into a musical designed to be performed by high school students. The show's Villain Song is titled "It Feels So Good To Be Bad".
Queen Fay in Overlord II, after her corruption via the Overlord's Magic while trying to perform a Heroic Sacrifice for her people by sacrificing her energy to power the Tower Heart and letting the Overlord drain her magic. As the process continues, she begins to accept that Evil Feels Good and embraces her Fallen Hero status.
In Persona 2, a strange force starts causing a segment of the population to start transforming into Jokers, vicious white-skinned, ruby-lipped killers with paranormal abilities. This even extends to one of the party members, and when attacking during her boss fight, one of her quotes is "It feels good!"
All incarnations of Doctor Robotnik from Sonic the Hedgehog seem to KNOW that they're bad, and generally seem to have a kickass time doing so.
In Rey Mysterio's Road to Wrestlemania storyline in WWE Smackdown vs. Raw 2011, once he regains his memory, you can choose to have him go back to being a Face or have a full-blown Face-Heel Turn. If you choose the latter, he reveals that he actually got his memory back a while ago, but played dumb because being a bad guy was so much fun.
The boss from Saints Row 2 flat out tells this to Julius, right before he/she blows his brains out. He/she's a complete sociopath by the sequel anyway.
Many of the assassins in the No More Heroes series seem to have fallen into this. Mainly Destroyman, who delights in every violent act he does.
Kotomine Kirei, from the same game, is a particularly extreme example. The only thing in the world that makes him feel fulfilled is the pain and suffering of his fellow human beings, whether he inflicted it or he's just observing it. To him, evil doesn't just feel good... it's the only thing that feels good.
Yet, as the Heaven's Feel route reveals, he still has a conscience and knows being evil is 'wrong'. The very fact that Evil Feels Good makes Being Evil Sucks for him, in a weird deconstruction of both tropes. He just can't win.
When Shiki in the Tsukihime game taps into his Superpowered Evil Side, he discovers to his horror that killing feels really good, and that he apparently wants to kill very much. In one possible path, he's finally informed that his Evil Side never wants to kill humans, only monsters and demons. In one possible Bad Ending, his psyche is so badly broken that he decides he really is a serial killer, and goes out to be just that.
Vampires also embrace this trope, especially Satsuki. Is possibly responsible for Shiki's issues with it.
KateModern: Precious Blood, when the phrase "The Hymn of One is fun" is chillingly given a whole new meaning.
Lenny Priestly in web RP Survival of the Fittest is this trope with what seems to beintense commitment to his twin sister Elizabeth. At first he was reluctant to kill, dreading the fact that he would have to in order to get his sister off the island, but after killing Anna Vaan he realizes that killing gives him a feeling of power he's never had before, and becomes addicted to that rush to the point where he attacks anyone he sees without any provocation with the excuse that they're threatening his sister. His reasoning is that if he's going to die to get her off the island, he might as well have fun. Ironically, his earlier actions come back to bite him hard when Gabe McCallum, furious with Lenny for one of his earlier murders, kills Elizabeth.
Maxwell Lombardi from V4 originally viewed dispatching his classmates as simply a necessity to get off the island. After killing Cody Jenkins (his 4th victim), he realises that he's actually starting to enjoy it instead.
The episode of The Tick where Arthur gets Baron Violence's belt pastiches this storyline.
Vlad Plasmius often uses this reason when egging Danny into the dark side in Danny Phantom. Since this is part of Danny's Myth Arc, he goes through several episodes where he feels good using his powers for dubious reasons. Mind controlled or not, in "Control Freaks", Danny had himself a ball!
Most alarmingly, Ron is an effective and successful villain; making Doctor Drakken look sick, even on the blue one's best day. He certainly had Shego coming to heel; quite an achievement, and one that scared her into getting Dr D back.
The second time he was turned evil (by an upgraded version of the same device), he proved to be insanely competent at martial arts, to the point where his prowess surpassed Kim herself. This adds more fuel to the theory that he subconsciously holds himself back in order to not steal the spotlight from Kim.
The very concept of schadenfreude. The word originates in German and means "mishap-joy", enjoying of misfortune of the others. People with high oxytocin levels are liable to schadenfreude.
There is a debate that goes in the field of Psychology that Anger and other negative things can act as a form of Catharsis. Which can be understandable as one is repressing such actions they once took throughout their lives. It may also be considered a mental defect in most fields as a result of a neurochemical overreaction; The thrill of doing such things brings a rush.
Psychopaths feel no remorse and are not afraid of being punished, and thus indulge every whim they have, not caring about what happens to others. In fact, doing criminal things often gives them a rush that many become addicted to.
We have laws. There are a multitude of theories why we need them. Do humans have such a tendency towards animalistic savagery, that we need to keep them in check? Does the influence of society corrupt us so much that, without them, we would mistake good for evil? Are we all just crazy and stupid? Or is it just as simple an idea as perfectly sane rational people are just tempted to screw over their fellow beings for any temporary advantage because it feels good.
Averted with one of the biggest, one might say, deadliest sins: Envy, which had never made anyone happier.
Played straight when the target of envy suffers leading to a schadenfreude.