Murtagh from Inheritance Cycle was derailed from a sympathetic villain who works for the Big Bad in order to make the world a better place to a one-dimensional snickering cliche that rivals Snidely Whiplash. This could be considered a Take That to how he was becoming something of a Draco in Leather Pants to certain sections of the Hatedom.
It doesn't help that Galbatorix made it impossible for Murtagh to convert to Eragon's side. Really, someone who physically can't choose for himself which side he fights for is going to be miles more sympathetic than someone who can choose and mindlessly calls a person evil when that person, again, has no choice in the matter.
Victor Helios in Dean Koontz's Frankenstein series first desires to eliminate humanity and replace it with a masterrace of his own creation, and eventually take over the world and the universe. In the sequel series, his clone who has his memories and personality now just wants to wipe out humanity and then kill himself when it is completed for some reason. This is explained by Dean Koontz as the next logical step to the original motivation, even though they are two completely different things.
Artemis in the book Acheron, one of the most recent books in Sherrilyn Kenyon's Dark Hunter series starts out as a friendly and curious goddess at the beginning of the book. At the end, you just want to smack her for her unadulterated meanie abilities.
Older Than Steam: Satan in John Milton's Paradise Lost initially views corrupting mankind as a continuation of the fight for freedom from God. When he realizes that his rebellion against God has been a terrible mistake from the start, he decides that he can no longer be good and must embrace evil as his good. Later on, his motives further degrade to something akin to "Since I'm evil now, I must cause God as much trouble as possible." This decay is likely deliberate, as the whole arc of Satan being diminished in mind (as well as stature — from towering angel to serpent) exemplifies just how destructive evil can be to its practitioners.
Related to the Satan example is Sauron in Tolkien's legendarium — he starts out as a clever, manipulative Magnificent Bastard in The Silmarillion, who works with Morgoth only as part of his agenda to bring order to the world, then gradually degrades to the Kill 'em All type seen in The Lord of the Rings. This was a deliberate piece of decay on Tolkien's part, due to his belief that evil usually starts out with some kind of high-minded ideal but its methods (and Sauron's multiple deaths) eventually ruin its practitioners, leaving them arrogant, hate-filled, mindless destroyers.
Even more pronounced is what happened to Sauron's former boss, Morgoth (who is intended to be Satan). He started out wanting to create and be the God of his own universe. But as he realized that nobody but God could actually be omnipotent, he gradually descended into a pure Omnicidal Maniac, with a corresponding loss of his power as the mightiest angel. At least Sauron kept the same basic goal (wanting to rule the world) in mind the whole time, even if he reason shifted from "I'm the only one who can run things right," to "UNLIMITED POWER!".
Played straight as an arrow in the ongoing Star Wars Expanded UniverseLegacy of the Force book series. In the first book of the series, Jacen Solo has visions of galaxy-wide destruction, including his dueling and killing Luke, that can only be prevented by accepting the teachings of the Sith. Lumiya, a former Vader apprentice, softens the blow by teaching Jacen that through careful discipline and sacrifice, he can avoid falling into darkness like Vader and the Emperor. However, with each successive book, his altruistic motives are shown less and less, slowly replaced by ever-increasing anger, distrust, intolerance, and desire for power. All of the culminates when he kills Mara Jade, who had caught onto his downfall, for a reason (or at least a rationalization) that makes almost zero sense and was later completely abandoned by the series. In a more recent book, he Force-chokes a young lieutenant to death for a perceived error, despite the fact she followed all standard procedures and was commended earlier in the book for her attention to detail. At this point, almost all trace of his benevolent motives are gone and all that remains is the Dark Side.
As others have noted, Motive Decay seems to be an occupational hazard of Sith Lords. Every Sith whose motives we've had an opportunity to examine started down the "dark path" for reasons that would make sense to normal people. Some want to unite the galaxy (decent motive), or to protect loved ones (ditto). Even the worst 'merely' want personal power- which isn't an inexcusable sin in and of itself. And yet every Sith we've seen has degenerated into committing acts of sheer evil. This suggests that the trope in question is intrinsic to the Star Wars concept of The Dark Side. In the Star Wars galaxy, a Sith's motives decay themselves.
A common fan theory (probably the result of an accidental Expansion Pack Past in the Expanded Universe) is that even Palpatine had sympathetic motives at some point—he wants the galaxy united because he's foreseen the Yuuzhan Vong invasion. This is something of a cart-before-the-horse situation.
Emperor Jagang in the Sword of Truth books starts out as a man who believes in human superiority and unlimited potential, and believes that magic prevents the men from creating a technological society. Later, he is a religious fanatic/Omnicidal Maniac who believes that all men are evil and unworthy of life, and it's his defeat which causes a sudden technological development. At the point of his change, the books take a strong right turn into Objectivist didacticism.
Reversed in the Vorkosigan Saga, where the Cetagandans were introduced as pretty generic bad guys, defined by militarism and expansionism, but were soon developed as a society with hints of a history, changing goals, and internal disagreements.
In the Heralds of Valdemar series, this explicitly happens to Big Bad Ma'ar, alongside and combined with his Villain Decay. In the Mage Wars he was a Well-Intentioned Extremist who united the barbarian tribes and sought to impose a Utopia Justifies the Means. Through successive reincarnations and overuse of Blood Magic, he eventually devolves into a twisted, megalomaniacal schemer whose grandiose plans to Take Over the World end up thwarted by generation after generation of heroes. This all turns out to have been a scheme of the gods, who needed his knowledge to avert the return of the magical Cataclysm that started with his original death.
Horus's descent into evil is depicted as being triggered partially by a string of unfortunate coincidences and partially as a result of going from "I will save the Galaxy" to "I will rule the Galaxy" to "I will destroy the Galaxy". His initial actions are triggered by the Emperor's near suicidal mysteriousness about what his intentions are once the Galaxy is united under his rule. Horus, not knowing what the Emperor plans to do and having been made aware of how the Space Marines are being increasingly sidelined, rejects this and accepts the idea that as superior beings, the Space Marines have a right to rule. By the time of the Siege of Terra however Horus seems to have accepted the Chaos gods and is mostly in it for horrendously evil gits and shiggles.
Konrad Curze, the Night Haunter, started off as a Well-Intentioned Extremist who believed in maintaining order through fear, tenets he passed on to his Legion. Curze went mad after the Heresy and let himself be assassinated to prove that the Imperium was no different from him. In the years since, his Legion has mostly forgotten his ideals and spreads terror For the Evulz.
Magnus the Red was devoted to advancing humanity through the acquisition of knowledge. Unfortunately he had a taste for forbidden knowledge. Magnus was forced to join with Horus to survive the Emperor's wrath, and since then he's been obsessed with getting revenge on the Space Wolves that burned his planet.
Fulgrim started off pursuing perfection, but even before the Heresy he'd become an arrogant Jerkass who believed he represented perfection. Chaos played on his arrogance and twisted him into a hedonist. He's only gotten worse since then.
Alpharius Omegon was revealed in the book Legion to have sided with Horus to ensure that Chaos ultimately lost the war. Since he ran his Legion with absolute secrecy, nobody's sure if they've suffered Motive Decay or not following his death. For that matter, it's not entirely clear whether either of them is dead.
The Chaos Gods, like the Dark Side, are quite fond of decaying the motives of the fools who sign up with them willingly.
Thief from 8-Bit Theater stole only to pay for his father's medicine. After the whole issue was resolved, he kept being as greedy as ever. When pressed by Black Mage, he was asked "So shouldn't you, y'know, stop stealing?", to which the answer was a simple "No." Every single Elf is a total bastard. It's even part of their (stolen) national anthem.
Nicholas from The Care Bears Movie just wanted to cast a spell over the two remaining people in the world to get them to feel hatred. In the Dwedit Jamez Bond flash movie, he now wants to take over the world.
Justified both in-universe and out. Marik has never been quite sure of exactly what he wants to do to Yugi, and the show ultimately has to follow the basics of the original plot, albeit barely.
Marik: "No, I don't want to kill you. I just want to destroy you a little is all."
This video, which is a loose history of the Soviet Union and post-Soviet Russia (as told through the eyes of one humble worker) shows this happening to the speaker. Initially he rebels against the cruelties of the Tsar to create "a regime of peace and love" but just a few stanzas later after the defeat of the Nazis, he celebrates the thought of nuclear war with the US. The decay in his motives however is truly complete when the Soviet state falls and he enriches himself on the free market.
Now the markets are free/so much money for me/tell me why should I care for peace and love?!
Note that all around him the stage and background are going haywire and falling apart, (corresponding to the condition of Yeltsin era Russia) and yet he doesn't care about the fact that conditions for the people are essentially the same as they were under the Tsar, except that he's now the one with the money now.
To find the half-life of motive decay, we multiply the number of writers, times time, times the number of characters. Know that different characters decay at different rates. Now turn your books to page 42.