There are several well-known monarchs whom I'm calling "unexpected kings"; the reason being that they had elder brothers. Only the death of the sibling suddenly elevated them to the status of heir to the throne.
Richard I of England, the great crusading warrior nicknamed Richard the Lion-Heart, was only the second son of Henry II. Until Richard was 26 the heir was his elder brother Henry, who was given the title of "The Young King". Henry, Richard and their younger brothers Geoffrey and John all rose in concerted rebellion against their father; but then young Henry died childless in 1183, leaving Richard to succeed to the throne in 1189.
Richard himself was childless when he was killed in battle in 1199. The next brother, Geoffrey, Duke of Brittany, was already dead, and by modern rules the heir to the throne should have been Geoffrey's 12-year-old son, Arthur; but rules of strict hereditary did not yet apply, and rather than run the risks involved in having a child as king, John, the youngest of the four brothers, was given the crown. Arthur was taken prisoner, and John apparently had him murdered in 1203. John is remembered as one of England's worst Kings, but would he have become King if Richard had lived a few years longer, until Arthur became an adult?
We might consider the case of Edward II (reigned 1307-27), another spectacularly incompetent King. He he was only a few months old when his elder brother Alfonzo died from unknown causes in 1284. Very few English people realize that they might have had a King Alfonzo!
A much more famous case is that of Henry VIII. He was born in 1491; five years after his brother, Prince Arthur. At the age of just 15 Arthur was given a diplomatically-important marriage to the equally young Katherine of Aragon, the daughter of Ferdinand and Isabella, the King and Queen of Spain. Arthur died a few months later, and when Henry VIII succeeded to the throne in 1509 he decided to marry Katherine himself. This had enormous consequences for the history of England, because marriage to a deceased brother's widow appeared to be forbidden in the Bible (the book of Leviticus). Henry therefore had to seek permission of the Pope for his wedding to take place. This was duly given, but when after 20 years Katherine had failed to give him a son and heir, Henry became convinced that his marriage was cursed, and sought to end it (tecnhically by annulment, not divorce). Unfortunately at this point European politics intervened, for in 1527 Rome fell to the forces of the Emperor Charles V, and the Pope (Clement VII) was a virtual prisoner of the Emperor. As it happened, Katherine was the Emperor's aunt, and there was no way Charles would permit this tremendous insult to his family; so the only solution Henry could find to his dilemma was to separate the English church from Rome and end the marriage by his own authority.
English Kings had been murdered or killed in battle, but the unfortunate Charles I was the only one ever to be sentenced to death and executed. But until he was 12 years old he had an elder brother, the popular Henry, Prince of Wales, who died at the age of 17. Would the English civil war have happened if Henry had become King instead of Charles?
More recently, as a young man George V was not expected to become King, because the eldest son of the future King Edward VII was the Duke of Clarence, Prince Albert Victor, commonly known as Prince Eddy. He was a dissolute young man; he was rumoured to be a regular visitor to a discreet homosexual brothel staffed by working-class teenage rent-boys, and has even been suspected of being Jack the Ripper! It was thought that the only hope was to find him a strong-minded and sensible wife, and a German princess, Mary of Teck, was selected for the job. But in 1892, before the marriage could take place, and doubtless to everyone's secret relief, Prince Eddy died of pneumonia at the age of 28, leaving his brother George as heir. It was tactfully discovered that Mary's affections were transferred to George, and the pair were duly married the next year. As King (1910-36) George V faced several serious crises, but always behaved with the strictest constitutional propiety, and the monarchy survived and prospered. Quite probably Prince Eddy would have done far worse.
King Louis VII of France (reigned 1137-80) was originally intended for the Church, and it would have been better if he could have stayed there. Unfortunately for France, his elder brother Philip had died as a teenager in 1131. Louis was noted for his piety, but as a warrior was an extremely inept crusader, and his main achievement as King was to divorce his wife, Eleanor of Aquitaine. She then ran off with the future Henry II of England, taking her vast holdings in south-western France into the English orbit. Louis was one of the worst of the French mediaeval kings, but fortunately for his country his son by his third wife, Philip II, "Augustus", was easily the best, and recovered the ground that his father had lost.
Francis I of France (reigned 1515-47), a dazzling prince of the Renaissance, had trained his eldest son, Francis the Dauphin, to succeed him, but the young man died at the age of 19 in 1536, and the succession went to the younger son, who became Henry III. Henry had no more than average ability, and was killed in a tournament in 1559. His three sons succeeded him in turns as King. All were disatrous failures, as France was engulfed in a violent civil conflict known as the "Wars of religion". None succeeded in producing an heir, and the Bourbon dynasty came to an end in 1589.
When the Russian Tsar Alexander II was assassinated by a terrorist bomb in 1881 and succeeded by his son, Alexander III, it was commented that the assassins (a small anarchist-populist group knwn as "People's Will") had murdered an intelligent liberal Tsar in order to replace him with a stupid reactionary one. But until Alexander III was 20 he would not have expected to become Tsar, because it was only in 1865 that his elder brother Nicholas died unmarried. Alexander III himself died young, at the age of 49, and was succeeded by his ill-prepared son Nicholas II. Nicholas as a child had witnessed his grandfather's murder: one can only guess at the traumatic effect it must have had on him.
There has been quite a fad in recent years of "virtual" or "alternative" history, asking whether things would have developed differently if there had been some accidental change - in the examples above; what if the elder brothers had lived? This leads to a more general debate: how much do individuals really matter in history?