Not all Big Bads start their lives evil. Sometimes, that shift occurs while the story is happening. Big Bad Slippage happens when a good or not evil character becomes more antagonistic/villainous over the course of the story, and their actions throughout the story ultimately make them a Big Bad. These characters may not be obvious at first glance. Note that they do not have to be evil. This character may just become an Anti-Villain or a Well-Intentioned Extremist.
Subtrope of FaceHeel Turn. Can also coincide with Slowly Slipping Into Evil. When a rebel leader undergoes this, it's also a case of The Paragon Always Rebels. Can coincide with Then Let Me Be Evil, when a character who has been treated as evil, regardless of their actions, says "screw it" and decides to meet everyone's expectations by being evil. Compare Start of Darkness, which shows how the character became evil via a prequel story or flashbacks. Compare Protagonist Journey to Villain, when the main protagonist crosses over to the dark side (but not necessarily become a Big Bad) over the course of the story. Contrast Evil All Along, where the character is always evil rather than just slipping into evil. Inverted by Ex-Big Bad, in which a permanently defeated Big Bad continues to be a non-Big Bad character in the story instead of being killed, imprisoned, or otherwise written out.
This trope occurring to a character at all is often a spoiler. Beware of unmarked spoilers below.
- The Black Wizard Zeref from Fairy Tail is built up over the whole story as being an evil Greater-Scope Villain who will become the Big Bad when resurrected/unsealed. When it turns out that he was not only alive and not sealed, but a man who hates his own crimes, his Big Bad status seems to be subverted. However, his very presence makes him an antagonist anyway, and as things die around him because of his magic and as he sees how Humans Are Bastards, he begins deliberately using his powers to kill anyone he sees as evil. Eventually he decides to become the evil man that everyone thought he was and becomes a full on antagonist for the rest of the series.
- In the second season of Tengen Toppa Gurren Lagann, Rossiu slowly becomes a Well-Intentioned Extremist, even having Simon imprisoned and sentenced to execution. When he is reminded of the way the heroes solve their problems, he goes back to normal.
- For about half of [C] - Control, Souichirou Mikuni is the Big Good who mentors Kimimaro Yoga and runs an organization that protects people from the Reality Warper powers of Midas Bank. Then he strikes a deal with them and floods Japan with their reality-warping Midas Money, which alleviates Japans economic problems for the present but allows Midas Bank to cause even more trouble, brings inflation, puts people out of jobs, and causes mass Ret Gones. Kimimaro is then forced to oppose Mikuni to restore Japans future.
- The Flash has Hunter Zolomon, the second Reverse-Flash/Zoom who was initially introduced as a simple supporting character in the Keystone police department far into then Flash Wally West's superhero career and developed throughout years in friendship role, until his shocking FaceHeel Turn. And even that said heel turn happens almost a year after when his Start of Darkness begins upon being paralyzed by Grodd.
- In the backstory of Runaways, Tristan and Lillie were superheroes back in the early 1900s, but a gang war changed their lives forever and left Tristan horribly disfigured. They spent the next century scheming to develop Time Travel and undo the damage. Of course, what they fail to realize is that when they send the Runaways back in time to bring their younger selves into the present day in order to let them escape the gang war, they just perpetuate a Stable Time Loop in which the Runaways themselves spark the gang war.
- The Transformers: Lost Light does this with the character of Getaway. At first Getaway seems to be just another member of the Lost Light's crew, engaging in all the wacky hijinks. Then he is disgruntled with the way the quest the ship is supposed to be on keeping getting interrupted and hates the idea that former Megalomaniacal Dictator Megatron is now a repentant member of the crew, then he starts plotting mutinies, wiping people's memories, putting people in a coma to trap them in memory loops so they keep repeating the same sequence of events over and over in their minds, making deals with serial killers, and tossing people into the robot piranha pit for the crime getting in his way or spoiling his plans. Not to forget, declaring himself as the next Future Prime of Cybertron and monologuing about his destiny.
- Disney's Beauty and the Beast gives us Gaston, the village hero whose worst deeds at first involve forcing a wedding proposal onto Belle, but doesn't seem like anything more than a pompous buffoon. But when Belle's father begins ranting about a beast that kidnapped his daughter, Gaston shows how low he can sink by threatening to declare him legally insane unless Belle accepts his proposal. His transformation becomes complete when he learns of Belle's affections for the Beast, causing him to rally an angry mob to kill the now sympathetic "monster" for even daring to take Belle away from him.
- Megamind starts off as a Villain Protagonist Big Bad. When he gets bored after seemingly (but not really) killing his arch-nemisis Metro Man and taking over Metro City, he decides he needs a new nemesis and creates one out of Hal Stewert, attempting to shape him into a superhero known as Titan. However, Wrong Genre Savvy Hal believes that getting superpowers will automatically cause Roxanne to fall in love with him, but when she still rejects him, he decides to be a super villain instead. This is what leads to Megamind's HeelFace Turn partway through the movie.
- Douche from Sausage Party starts off as, well, a douche. As he grows more consumed with revenge against Frank for ruining his chances of getting used, however, he starts outright murdering other drinks and draining their liquids to gain power, clearly (and literally) drunk with power.
- The Amazing Spider Man 2 has both members of the Big Bad Duumvirate go through this.
- Harry Osborn starts off as a friend to Peter Parker. However, Harry's stress over his disease that will kill him causes him to lose his sanity over the course of the movie and he becomes more antagonistic to Peter. This comes to a head when he teams up with Electro and injects himself with a serum that turns him into the Green Goblin.
- Max Dillon is initially a timid worker at Oscorp who's The Chew Toy until he gets into an accident of falling into a pool full of electric eels while at work (he's the engineer). After that, he becomes Electro, and he's not a baddie yet... then he goes to the Times Square at night where people are scared of him... but he's happy that people start recognizing him. Up until a wayward sniper shoots him, making his electric powers go out of control and Spider-Man trying to suppress him; people start cheering for Spidey, and this makes Electro deem him his enemy. From then on, he becomes one of the villains.
- Maleficent has King Stefan, who, while kicking off the story by stealing Maleficent's wings and sending Aurora away, is not exactly evil at those points. By the end of the story, however, paranoia has broken him and he tries to kill Maleficent even though she just saved his daughter.
- Loki from Thor is the title character's apparent brother and starts off as mostly good but jealous. He is revealed to be a Frost Giant partway through the film and that he let the Frost Giants into Asgard earlier in the film so he could stop them himself out of jealousy over Thor. When the reveal that he's a Frost Giant comes, he decides to take over Asgard. He manipulates King Laufey into helping him, pretending they're a Big Bad Duumvirate, and betrays him at the end of the film, leaving Loki as the sole Big Bad.
- In Gormenghast, Steerpike begins as a rebel dissatisfied with the lowly position allocated to him at birth. He runs out on his assigned job as a kitchen scullion and sets about metaphorically and literally climbing to a higher social station, despite the law and tradition of the city-state. In the beginning, the reader can sympathise with him for his ambition, for wanting to break with stultifying convention, for wanting to get a satisfying position in line with his intelligence and talent. An occasional lie and a bit of confidence trickery can be forgiven. But then people start to die. The sympathetic young rebel is becoming a monster. He finally dies as a scarred and deformed monster, having murdered the heroine Fuchsia and been hunted down by her brother.
- In A Tale of Two Cities, Madame De'farge becomes this. Where in the beginning of the novel the first conversations with her present her as an ordinary woman, albeit with some relatively extremist views, when the revolution takes hold and France is thrown into chaos, she eventually becomes a strong leader in the revolution and is among the front runners in the category of "sending everyone with a shiny hat to their deaths."
- In Shadows of the Apt, Seda is introduced as the timid princess of the Wasps who lives in constant fear that her older brother Emperor Alvdan will someday have her assassinated like he did their other siblings. This starts to change when Alvdan's advisor Uctebri takes Seda under his wing, at which point she gains knowledge of Blood Magic and a significant injection of confidence. At this stage, she starts demonstrating her true competence and ambition, and ultimately outlasts both Alvdan and Uctebri. Seda ends up becoming an immensely powerful magician as well as Empress, and holding the throne of Big Bad for most of the second half of the series.
- Although Loki is probably the Ur-Example if he wasn't always a Devil in Plain Sight, the trope is explicitly referenced in Hannah More's 1799 work Strictures on the Modern System of Female Education.
There are, however, multitudes of the Young and well-disposed, who have as yet taken no decided part, who are just launching on the ocean of life, just about to lose their own right convictions, virtually preparing to counteract their better propensities, and unreluctantly yielding themselves to be carried down the tides of popular practices, sanguine, thoughtless and confident of safety. - To these the Author would gently hint, that, when once embarked, it will no longer be easy to say to their passions, or even to their principles "Thus far shall ye go, and no further." Their struggles will grow fainter, their resistance will become feebler, till born down by the confluence of example, temptation, apetite and habit, resistance and opposition will soon be the only things of which she will learn to be ashamed.
- Lex Luthor from Smallville begins the show as Clark's friend, gradually becomes increasingly paranoid and controlling, ends up as a villain sometime around Seasons 4/5, and is the Big Bad of 6 and 7 (and a Disc-One Final Boss in 8).
- Slade Wilson in Arrow starts off being stuck on the island with Oliver. He helps train Oliver to make him into a skilled fighter. After being badly burned, Oliver, Shado, and Sara use the drug Mirakuru to save him. It works, but there are nasty side-effects that cause Slade to be mentally unstable. Soon after giving him the drug, Oliver, Shado, and Sara are captured and Oliver is forced to choose to save one of them; he chose Sara. When Slade finds out, he is dead-set on destroying Oliver in every way possible.
- Jiaying from Agents of S.H.I.E.L.D. is in charge of Afterlife and the head figure of the Inhumans there. While she is rather nice at first (especially to her daughter, Skye), her nasty side begins creating a lot of conflict concerning the Inhumans, to the point where Jiaying kills Agent Gonzales and starts a war between the Inhumans and S.H.I.E.L.D.. And in a subversion, she's actually behind the actions of Cal, making her the Big Bad the whole time. It turns out that she actually was a nice person to begin with, but was vivisected by Dr. Whitehall and Came Back Wrong thanks to Cal's efforts to save her.
- On Grimm, Adalind removes Nick's powers through a magic ritual. Juliette agrees to restore them through another ritual, but it has the side-effect of turning her into a Hexenbiest. Then she starts to have a complete morality breakdown, leading to her starting fights in bars using her new powers, setting fire to the trailer where Nick keeps his journals, and conspiring with the villains to kidnap Adalind's daughter. That last act also leads to the death of Nick's mother. And leads to her being Killed Off for Real... apparently.
- There's debate about whether Warren or Willow is the true Big Bad of Buffy the Vampire Slayer Season Six, but both of them are examples of this. Willow is the hero's best friend who briefly slips into extreme evil at the end of the season due to a Trauma Conga Line exaggerating her personality flaws, while Warren starts off in the previous season as morally neutral but weak, and gradually becomes more and more evil over the course of the sixth.
- Barry Allen is the hero of The Flash (2014), but the Big Bad of the third season, Savitar, is a time remnant of him from the future. Savitar has not only suffered all the trauma of his past self, but was also ostracized and abandoned by his friends and family for not being the "real Barry", too. As a result, he has dedicated himself to ruining the lives of Team Flash and bending history itself to his will.
- Jax from Sons of Anarchy starts out as a light idealistic character who is always looking for the least violent way to achieve club goals. However, as more and more impossible demands are placed on the club, he more and more turns to the violent methods he once disagreed with, in order to keep his family and friends safe. Following the the murder of Opie, his best friend, Jax takes a very dark turn, fixated on revenge. When Tara, his wife, is murdered, Jax completely spins out, to the point of tunnel vision, becoming the villain he never wanted to be.
- Technically, nearly all the villains from Once Upon a Time are examples of this, starting out as fairly nice people and then getting slowly broken until they become monstrous. The main antagonist for most of Season 5A is the one we see go through this during the course of the show. She's Emma Swan, the main protagonist after she became the Dark One. After spending weeks trying to resist, she starts to do some morally ambiguous things, and when Hook is fatally injured, she completely snaps and lets the darkness overwhelm her and almost lets Hook join her.
- The last three episodes of Series 9 of Doctor Who is essentially about doing this to the Doctor. Twelve is not the most friendly Doctor at the best of times, but there is a heart of gold deep down that will protect his companion no matter what. So when the relationship between him and Clara starts to turn unhealthily co-dependent and an action leaves him unable to save her when she is abut to be killed, he very nearly snaps, but is stopped by Clara's last wish. A couple of billion years in a torture chamber immediately after changes that, and he essentially becomes the Final Boss of the series.
- Despite being one of the most famous examples of a Villain Protagonist, Macbeth is simply a good soldier and lord at the beginning of the play. It's not until the trio of Wicked Witches and his own wife Lady Macbeth push him to murder King Duncan and take the crown for himself that he even considers the idea. But once he does usurp the throne of Scotland, he becomes a ruthless tyrant far worse than his wife, and the witches are forced (by Hecate) to stop him.
- In Fire Emblem Fates: Conquest, one of the main antagonists is Takumi, your younger brother on the Hoshido side. He's easily the most Recurring Boss of the Hoshido siblings, fought as many as five times compared to elder siblings Ryoma and Hinoka, tied with three if you count the battle at the start of the route split. Each time, he only gets more and more violently enraged with you and the other Nohrian Scum's general existence and "betrayal," acting more antagonistic overall than anyone else in Hoshido, even returning from his presumed suicide and acting as the Final Boss of the route even after you've defeated King Garon and his cronies. This is partially the result of the influence of the Greater-Scope Villain, though he was only able to do this so extensively due to Takumi's own anger being unchecked.
- Mega Man Zero 2. This game introduces Elpizo, the new Resistance leader (while the old one, Ciel, goes on to work more on her research). After Elpizo and Resistance's botched assault on the Neo Arcadian empire, he turns bad and goes on a quest to find the Baby Elves to grant him more power. Later, you'll meet him as the Final Boss.
- Kenny from Season Two of The Walking Dead was your ally mere episodes earlier but his temper and Sanity Slippage slowly turned him into a Big Bad, while Jane, his main rival for the position at this point, came across as comparatively level-headed and affectionate towards Clementine, if rather aloof and callous otherwise, only to turn out to be equally flawed.
- Beyond: Two Souls: Nathan. We get to witness signs of his gradual slippage during the course of the story, even though the most notable stages of it are revealed in the last stages (partly due to the game's disjointed narrative).
- Warcraft III's human campaign has Prince Arthas Menethil gradually taking more and more desperate and antiheroic measures in his efforts to save Lordaeron from the Scourge. This ultimately results in him taking up the cursed blade Frostmourne in Northrend and becoming a death knight in service of the Scourge.
- The sequel World of Warcraft continues the trend with Garrosh Hellscream, who is introduced in Burning Crusade as a minor character and gets brought back in Wrath of the Lich King as an ally, if a somewhat jerky one. Then he replaces Thrall as the Horde's Warchief. Cue him growing progressively more tyrannical, unlikable, racist, and pointlessly hostile toward the Alliance, eventually leading to the other Horde leaders all rebelling against him and teaming up with the Alliance to take him down.
- StarCraft's Terran campaign has Arcturus Mengsk start out as a Rebel Leader and the closest thing the campaign has to a Big Good, only to gradually use more questionable methods to overthrow the corrupt Confederacy, culminating in him unleashing a Zerg invasion on their home planet and leaving his most faithful lieutnant to die with them. Cue Raynor quitting out of disgust and Mengsk proclaiming himself The Emperor, setting him up as the main human villain of the franchise.
- The Shin Megami Tensei series generally starts with their major antagonists already pretty rotten, but there is one notable exception: Zayin of Shin Megami Tensei II. Initially a simple elite Temple Knight following the Center who happens to have a strict moral code, Zayin turns on the Center upon realizing how corrupt their leaders are. After trying to stop the Archangels in charge and being saved by Aleph, he takes over the Center as a more benevolent figure. Things go downhill when he tries to force Aleph to join him and, if refused, kicks him out of Eden. One of the game's main twists is soon revealed: Zayin, not Aleph, is God's Chosen One, and God isn't that good himself. Zayin eventually fuses with Seth to become Satan and leads the Law faction in God's name. On Neutral and Chaos, he doesn't recover, but if Law is picked, he ends up regaining his morals and fights YHVH alongside Aleph, dying in the process.
- Randal's Monday: Sally eventually becomes the Big Bad after she gets the ring. She's with it so long that she gains its powers in full.
- In Live A Live, the Wham Episode Medieval chapter focuses on this. Your Player Character Oersted starts off as a generic Heroic Mime Knight in Shining Armor. However, repeatedly being betrayed and abandoned by his friends begins to do a number on his sanity. When his Love Interest Alicia professes her love for the man responsible for orchestrating Oersted's downfall and commits suicide, Oersted finally snaps and transforms into the game's Overarching Villain Odio.
- Played with in Crisis Core. Sephiroth's main character arc is him slowly slipping from Shinra's greatest SOLDIER to the Big Bad of the Compilation of Final Fantasy VII. However, he never becomes more than a peripheral antagonist to Zack, whose plotline focuses on and climaxes with his conflict with Genesis.
- In the final season of The Legend of Korra, Kuvira starts out as a harmless background character who helps save Korra's father in the season 3 finale. However, she is later put in charge of stabilizing the Earth Kingdom after the assassination of the queen, and her fascist methods of unifying the country put her at odds with the main characters.
- There's a Man in the Woods: This short animated film tells a story of a man who gets fired from a teaching job because one of his pupils lies about seeing a serial killer near the playground, and then gradually turns more and more stressed-out and mentally unstable until he becomes a serial killer himself.
- Eddie Brock in The Spectacular Spider-Man starts out as Peter Parker's long-time best friend and a genuinely nice guy, only for a chain of events to lead him to become gradually more bitter toward both Spider-Man and Peter, culminating in his transformation into Venom.