Buu's internal conflict splits him into good and evil halves. While good Buu is far from impotent, he is still bested by his evil side and is absorbed into the more powerful Super Buu. Later on, when Buu is extricated from Super Buu, it is revealed that "Fat Buu" was created when the Grand Supreme Kai was absorbed by the original, pure evil, Kid Buu.
Earlier, Kami splits into a good and evil half to ensure that he could work as Earth's "God" (God in this series is actually a job position) without the evil in him causing him to abuse his power. The evil half that becomes Piccolo Daimou is not, as far as the audience can see, actually any stronger Kami, but Kami is a Non-Action Guy so it's rather academic. The 2nd Piccolo is stronger due to a combination of youthfulness and an inversion of Villain Forgot to Level Grind.
In one Slayers made-for-video movie, a magic mirror creates opposite clones of Lina Inverse and Naga. To the villain's horror, these turn out to be simpering, useless "good" copies that won't fight. This leads to his greater horror when he realises what this must make Lina.
The League of Extraordinary Gentlemen's Dr Jekyll and Mr Hyde are the embodiment of this trope, in one package. Hyde even explains to the rest of the League why he has been getting stronger and stronger while Jekyll had been getting correspondingly weaker: as the embodiment of Jekyll's id, Hyde has all the drive and ambition, leaving Jekyll with nothing.
In Watchmen, the superheroes come in three flavors: sympathetic and ineffectual, sociopathic and effectual, and Dr. Manhattan, who is all over the map depending on how "into" humanity he's feeling this week.
Better illustrated by the director's cut version of the line: "I ain't that good."
Superman was affected by a different kind of kryptonite, and winds splitting him self in to good who is Clark and bad as Superman, the good part of him took one heck of a beating in Superman III — for a while, and gets back up for a second wind.
In Me, Myself, and Irene, the whole reason Charlie's psyche creates his evil alter-ego Hank is because Charlie's too much of a nice guy to stand up for himself.
In The Dark Crystal, the wise and gentle urRu and the cruel Skeksis were split from a single race of beings, the urRu retaining their wisdom, benevolence and purpose but lacking the will to take action, while the Skeksis retain their strength, force and willfulness but lose their wise and compassionate natures and devolve into a Deadly Decadent Court.
Done very subtly in The Good, the Bad and the Ugly. Most of the characters and plot are stuck firmly into a Black and Gray Morality setting. The few truly good characters such as the priests or the Union Commandant are either helpless to change things or relegated to standing on the sidelines while the Black and Gray characters run things.
The Animorphs book The Separationaverts this with Rachel—neither sides are explicitly good or bad. They each got half of the components of her personality at random. The "good" side was given Rachel's tactical and strategic abilities, leaving the "evil" side with just the blood rage and ability to fight. The end of the book results in the evil Rachel grudgingly having to work alongside the good Rachel because evil Rachel can't plan or strategize worth squat, so is totally unprepared for any reaction to changing battle situations other than "kill something."
Subverted in A World Gone Mad. Wally's evil half is lazy, hedonistic, and has zero impulse control, spending all his time chasing women and watching TV. Wally's good half is proactive and driven to actually solve the problem at hand (to relieve the suffering of others). The good half probably has an aversion to violence, but since Wally's a stealthy-type "sneaker" that's not really an issue anyway.
Played with in Il visconte dimezzato by Italo Calvino. When the protagonist is split in two halves, one good and one evil, the evil side is a ruthless bastard, while the good side is kind and generous but rather ineffectual.
In The Lord of the Rings, the Fallen Angel Sauron is busy conquering and wrecking the place up while his good counterparts are limited to guiding and influencing the Free Peoples and are forbidden to try to match him strength-for-strength. This is less because they're weaker and more because the last time the Valar took on the Big Bad directly, the resulting battle leveled a few major continents.
In A Swiftly Tilting Planet, this is somewhat averted: the "good" people are proactive, strong, and willing to face evil, at least to give it a stern talking-to. But the wicked people are tolerated in the communities because they're better at things - one woman who is racist against Indians is the best midwife in the village, and Gedder the evil sumbitch is able to teach others how to farm.
Most any portrayal of a stereotypical angel. Ironic, since in Judeo-Christian scripture, angels are portrayed as warriors or executioners just as often as they are as messengers, and usually have to tell any humans who recognize what they are not to be afraid.
Lord Vetinari claims this to Vimes in Guards! Guards! ("A great rolling sea of evil ... but people like you put together little rafts of rules and vaguely good intentions and say, this is the opposite, this will triumph in the end, Amazing!"). Vimes's entire subsequent career proves him wrong.
This and Good Is Not Nice is what usually leads to Jedi purges. In many eras, Jedi candidates are harvested from infancy or early childhood, cut off from and forbidden "attachments" (family, love, and any close bonds) or strong emotions, cultivated in relative isolation in temples and enclaves, and restricted from anything that challenges the prevailing dogma unless they're Master rank. This is all to make it so the product of this system never succumbs to the Dark Side. Then, they're sent out into the galaxy into the thick of galactic conflict as fighters (who can't strike unless in defense), exposing them to things that would make Audie Murphy wince, while being forbidden the comforts that even a common soldier would get (like friends, lovers, or a family to come home to)...And then The Order wonders why so many of them snap and start carving up planets?
While it's not so much "good" as "good intentions," the Kindar have done such an awesome job of banishing conflict, crime, and even words like "anger," that their society is killing itself. Their Erdling counterparts are still hippies by our standards, but their acceptance of their emotional darker side makes them more robust.
The classic Star Trek episode ("The Enemy Within") where Kirk was divided into his "good" (peaceful, passive, pacifist) and "evil" (violent, malicious, aggressive) selves. What makes this the poster child for this Trope is that the "Evil" side does learn to calm down and even assumes command for a short time, while the "Good" side remains unable to make the simplest decisions (though downplayed in that the episode doesn't call it Kirk's good and evil sides, it just ends up that way based on the division of traits).
And both sides love Anya and try to protect her from the other one. Before they get put back together, Anya expresses an interest in trying out having two Xanders…
In the Red Dwarf episode "Angels & Demons", the ship Red Dwarf and everything on it was triplicated, but into good and evil versions. The good versions, of course, not only couldn't fight, but couldn't conceive of the idea of anyone intentionally causing harm. Needless to say, they didn't survive too long.
High Kryten: Poor devil must have a faulty gun. He's accidentally shot me five times. Oh, how I love him!
High Lister/Rimmer: "Apologies, brother. I seem to have stained thy knife with my blood. Allow me to furnish you with a fresh knife."
On American Gothic, no matter how hard Dr. Crower and Gail fight for Caleb's rights, and no matter how much Merlyn uses her angelic powers to protect him, Caleb is inexorably drawn into Buck's orbit and everyone seems helpless to prevent it, or even expose Buck's evil. It doesn't help that the sheriff is a Villain with Good Publicity and that both Matt and Gail are hardly immune to mind games or temptation, but even Merlyn is made out to be decidedly weaker than her adversary—and gaining more power to face him almost pushes her too far.
In Xenosaga, chaos all too often comes across as an indecisive wuss where as pretty much the whole series is a Evil Plan by his dark twin Wilhelm. Some people even suspect that Wilhelm?s true goal was to force chaos to make a decisive choice by attempting to make that choice for him.
In Tales of Legendia, Grune turns out to be the opposite force to Shwartz, but is obviously weaker, from her amnesia to her lacking the power to beat her archenemy by herself. (Something obviously happened to cause this, but the game never explains it.)
Tales of Symphonia: Dawn of the New World plays this mostly straight, as Emil is a cowardly little runt who can barely swing a sword while his "Ratatosk Mode" is a cruel and powerful Blood Knight. Subverted in the closing act of the game when Emil seals away his Ratatosk personality and turns out to be just as powerful. In fact, his Mystic Arte has been confirmed statistically to be more powerful than Ratatosk's even without the latter's extension.
Black & White. Good gods are harder to play than evil ones. They take longer to get set up and are much easier to impede. All of the useful miracles are considered evil to use, while the best you will manage with good is crappy shield miracles which an evil god can and will beat down easily.
On the other hand while evil is quicker and easier, patient builder Gods who only shoots the dogs when there aren't other options can amass more worshippers, resources, and a larger sphere of influence over time. Plus, the evil way of playing wastes resources (especially wood) that could be reinvested in your empire.
Implied in Star Wars: Knights of the Old Republic somewhat. In a gang war between the 'good' Hidden Beks and 'bad' Black Vulkars, the good guys are losing and they're the side you have to join. Also, the good guys are losing at the beginning of the game when the Endar Spire is overtaken by the Sith. Finally, Malak, the 'Dark Lord' and Big Bad of the game does a good job conquering most of the galaxy and has an army of loyal Mooks. Might makes wrong in Star Wars, it seems.
You can actually discuss this with Carth, who points out that Malak hasn't won yet and probably was only winning because of the foundation Revan built. It's also known that Revan was defeated, costing the Sith their competant leadership - Malak is so Stupid Evil that his tactical sense only goes as far as "burn the whole planet." Carth also remarks in the tomb of Ajunta Pal that the Dark Side's weakness is that it eventually consumes itself.
It also factors into the backstory, especially if The Exile is arguing with Atris or one of the other lost Masters. The Jedi were busy navel-gazing while Mandalore the Indomitable was leading his people on a rampage that got as far as Duro. Had Revan not disobeyed the Council by going to war, then the whole galaxy would have either speaking Mando'a, and/or been a satellite state for the True Sith.
There is a comic where a guy gets split into not two, but four parts of himself. Thus, there were two 'goods' and two 'evils'. But while one 'good' was indeed, rather wimpy, that part also had his rage. The other 'good' fought for justice, but had no sense of mercy. Of his 'evils', one was just a jerk, but was also the one with all of the humor, while the other 'evil' was bloodthirsty, but was also rather protective of his friends.
The Disney Afternoon cartoon Darkwing Duck featured an episode of this: Darkwing was hit by a beam that divided him into his "positrons" and "negatrons", resulting in him dividing in two. The "good" version was treacly, cloying, and so nonviolent he refused to even step on bugs... and of course useless in the effort to round up the evil version.
On top of that, the episode in question eventually averted this. Negatron-Darkwing gets hit with the splitter beam, which rather than splitting him again results in his evil getting even more concentrated, and suddenly he can shoot lightning bolts out of his fingers, throw cars with his mind, and cause earthquakes without even trying. It looks like he's given up on merely robbing banks and is about to destroy the city just to laugh while it burns, when in desperation Goslyn decides to zap the useless Positron-Darkwing with the splitter. What happens after that looks like stock footage from Sailor Moon, but it works.
Inverted in Xiaolin Showdown, where the good Jack Spicer is far more effective at fighting than he was when he was evil. Too bad he also annoys the hell out of the monks.
In one episode of the Mork and Mindy animated cartoon. You know the drill.
Done in Disney's Aladdin: The Series, but not with the main characters. Instead, they go to China and meet two identical twins, Zin and Zang, one evil and aggressive and the other good and meek. The twist is that neither has actual power until they merge into a dragon, and the alignment of the dragon depends on who forces the merge. When the evil one causes it, the dragon rampages and burns villages. When the good one finally grows a spine and takes control (which frightens the evil one for some reason), the dragon fixes everything.
Played with in Johnny Bravo when the villain of a Star Trek episode challenges Johnny to a battle between good and evil. He is assisted by two intimidating mooks, while Johnny is paired off with a choir boy who immediately runs away. Johnny still wins however, because the mooks are dumber than he is, and the villain is a wimpy geek who can't even slap him without hurting his wrist. What makes this weirder is that this episode happened after Johnny was turned into a bumbling idiot who screams like a girl and is beaten up by little boys on a regular basis.
Done in an episode of Kung Fu Panda: Legends of Awesomeness where Po, in his usual shenanigans, splits himself into his Yin and Yang half after glimpsing into the Mystical Mirror of Ying and Yang. While Bad Poe was ruthless, powerful, and even cunning (as when he sneaked up and imprisoned Shifu during his monologues where he realized Po's bizarre behavior was the result of the mirror), Good Po was overly flattering, nonviolent, extremely gullible (Bad Poe tricked him into trapping himself into a pillory just by asking him nicely) and even a little dumb (he thought he couldn't escape even though the keys Bad Po dropped were right in front of his feet and the pillory wasn't even locked in the first place as Good Po demonstrated when he retrieved them). Subverted in the end where Good Po intentionally undergoes multiple Amusing Injuries to weaken Bad Po so that the Furious Five will be able to make him gaze into the mirror again and reunite the two halves (since both Pos share each others' pain), then tricking him into destroying a replica while thrusting the real mirror into his face, regaining his old self.
Also comes up in an episode where one of the aforementioned Mirror Morality Machines starts being used randomly on a superhero team, causing the good and evil members to fight each other. For some reason, whenever one of the heroes is returned to being good, they immediately either start losing or try to run away.
Jainist monks in India don't wear clothes (so they can't crush insects in them), sweep the ground they're walking on (so they can't tread on bugs), and sometimes even wear a cloth before the mouth, so they can't suck in flies.
While less extremely ascetic, some Buddhist monks are also very careful to avoid harming even the smallest creature, carrying bells to warn insects and the like of their approach.
The Talmud teaches that without the "evil impulse", mankind would spend all of its time praying and studying God's Torah. Evil is therefore necessary, even good to a certain extent, because it makes people do good things like marry, build houses, and start businesses, which are all good because God wants human beings to create things on Earth.
This is something scorned in Islam. It's not enough to claim to be faithful: God shall test His servants. The test can come in many form: hunger, poverty, war... This is one of the reasons why Sufism (and the zuhud life-style) is derided. A Muslim can't simply lock himself in his home and spend all his time praying that God will solve all his problems. Amongst the very first verses of The Qur'an is about not wrapping yourself in Security Blanket (in Muhammad's case, he was cowering in fear after the first visitation by Gabriel).
The United Nations is almost universally recognized as an organization with noble aims, but is often criticized for being unable to get anything of substance done.