Unfit for Greatness

"Omnium consensu capax imperii, nisi imperasset."

- Tacitus, describing the Emperor Galba.

A fundamentally good and well-meaning character who attains (either by chance or through his own effort) a position of great prestige, power, and responsibility--only to cause more harm than good because he doesn't have enough willpower, foresight, and general virtue to handle them right. Despite trying to make life better for others with his power, he ends up wasting it on petty things or unwittingly pushing them into ruin. The tragedy of this character is that he often realizes that he screwed it all up but doesn't know how to fix it (and in this, he is different from someone who's simply Drunk with Power).

Such character may be contrasted by the one who is better suited for greatness but doesn't receive any recognition (at first). Both of them may want the same thing but by a twist of fate, the one who has the power is not the one who can pull it off.

Contrast Reluctant Ruler and Cincinnatus. See also Drunk with Power and Well-Intentioned Extremist. Reality Warping Is Not a Toy is a common fantastic variation.

Comic Books

 * Irredeemable is a giant extended exploration of this trope.
 * Magog, the Anti-Hero Substitute of Superman in Kingdom Come, turns out to be one of more well-meaning anti-heroes. He really only wanted to Make a Better World by killing The Joker and the like and before he realized he was wrong, he was filling in Supes' shoes, which eventually culminated in the Kansas disaster.
 * The one-shot JLA: Superpower is structured around this trope, in the person of Mark Antaeus.

Literature

 * Robert Baratheon from A Song of Ice and Fire fights valiantly in a just war to dethrone a mad king, but when he takes the throne himself, he proves himself incompetent in dealing with matters of state, spends lavish amounts of money on feasts and tournaments, and his inattentiveness to his own family sows seeds of disaster.
 * In Labyrinths of Echo, Chief of the police General Bubuta Bokh. He earned his rank and more for exploits during the war, remains loyal, not malicious (only noisy), his abuses of power are limited to petty embezzlement and nepotism. He's also completely unfit for this job, except the part when he scares arrested folk. Note that he doesn't actually screw anything up (yet)--if only thanks to the work of much more competent lieutenants working under him.
 * King Cinhil Haldane in the Legends of Camber trilogy. Having spent most of his life as a cloistered priest, he is unprepared for the machinations of politics, and is unable to prevent several human lords on his council from staging their own coup d'etat after his death. He also blames Camber (the man who engineered his succession to the throne) for the loss of his vocation and his misery over it, and he distances himself from an experienced courtier's advice when he needs it most.
 * Philipp Tagere from the Arcia Chronicles is a brave warrior and a charming diplomat but turns out to be a weak king who estranges his best courtiers and officers and surrounds himself with yes men. He is contrasted by his two younger brothers, Edmon and Alexander . On the other hand, he is also contrasted by Pierre Lumen, who  and is simply Drunk with Power and has an "It's All About Me" attitude when it comes to state affairs, completely blind to his lack of political skill. Pierre does approach this trope once or twice when he ponders on his and Alexander's differences but he prefers to suppress these thoughts.

Video Games

 * In Mass Effect 2, Jacob's disappeared father actually crash-landed with his crew on a planet whose environment began destroying their cognitive functions. At first, he rationed the uncontaminated food for the scientists working to get them all off the planet. Then, however, he gave in to temptation and turned the camp and mindless crewmates into his personal kingdom. By the time Shepard and crew arrive, things have gotten bad.