The most likely way a medieval monarch would be killed by a rival king would be on the battlefield. Examples include Harold Godwinsson (at the Battle of Hastings in 1066), Richard the Lionheart (killed at a siege in 1199), and John, King of Bohemia (who died in battle in 1346 after having been blind for ten years). However, this was fairly unusual, as, especially during the High Middle Ages, the rules of conduct meant it was much more profitable for a victor to take his enemy captive and then ransom him (the ransom of Richard the Lionheart in 1193 nearly bankrupted England, for example). If a king really wanted a rival king dead, it was often very difficult to justify publicly executing a divinely anointed monarch, so much so that ways were often found to murder kings without leaving marks on their body. Therefore, medieval kings were often sneaky about getting rid of enemies.

In England, there are several examples of kings who were done to death after they had been deposed: Edward II in 1327 (there is one particularly famous story as to how that happened), Richard II in 1400 (probably starved to death after a failed attempt to restore him to the throne), and Henry VI in 1471 (probably smothered after he was deposed for the second time). There are also two famous examples of English kings being involved in the disappearance/murder of people who had a better claim to the throne: firstly, King John probably ordered the murder of his sixteen-year-old nephew Arthur of Brittany in 1203, and, although it has been heavily debated, Richard III was probably involved in the disappearance of his nephews Edward V and Richard, Duke of York (more famously known as the "Princes in the Tower") in 1483.

There is only one example I can think of (in English history) where a monarch executes a monarch of another country, although it is a little out of our period. That is the case of Mary, Queen of Scots. In 1567, she was deposed as Queen of Scots (mostly because of her supposed involvement with her second husband's death, but also because she was a Catholic and most of her lords were Protestant). After a year in prison, she managed to escape and decided to seek refuge in England with Elizabeth I, hoping Elizabeth would help her restore her to her throne. Now, Elizabeth and Mary were related. Mary was the granddaughter of Margaret Tudor, who was the elder sister of Elizabeth's father, Henry VIII. As Henry had no surviving children other than Elizabeth (and it looked like Elizabeth would not have any children herself), Mary was the candidate most likely to succeed to the English throne after Elizabeth died. However, the same problems that had plagued Mary in Protestant Scotland troubled her in Protestant England. Elizabeth's Protestant ministers were very suspicious of letting a Catholic onto the English throne. Furthermore, the Scottish lords were anxious to present Elizabeth with evidence that Mary had been involved in her husband's death to legitimise their new regime, so Elizabeth was forced to launch an inquiry into Mary's guilt. Not really wanting Mary back on the throne of Scotland, but not really wanting her in England where she could be the figurehead for Catholic plots, Elizabeth prevaricated on what to do and ending up just leaving Mary under house arrest, which ultimately lasted from 1568 to 1587.

During her twenty years under house arrest, there were several plots to break Mary out and install her as Queen of England, but none that Elizabeth's spymasters could connect directly to Mary. That all changed in 1586 when Mary, probably at the end of her tether, joined a plot known as the Babington Plot. Although she wrote in code, a letter written by Mary, ordering a group of conspirators to assassinate Elizabeth, was found. Later in the year, Elizabeth arranged a trial for Mary where she was accused of treason, in spite of her not being an English subject. Nevertheless, she was found guilty and sentenced to death. However, Elizabeth had real qualms about killing an anointed monarch. Whether she just worried about how it looked to the wider world, or whether she truly believed it was a great sin is not known, but after putting it off for several weeks, she eventually signed Mary's death warrant and Mary was beheaded in 1587. In the end, Mary's death caused real trouble for Elizabeth, as the rest of Europe were horrified by what she had done, and it was one of the causes of the famous Spanish Armada of 1588. However, in the long run, it did solve the problem of England having a Catholic heir. When Mary had fled Scotland in 1568 she had left her two year old son, James, behind. In the absence of his parents, James was raised as a staunch Protestant by a series of Scottish noblemen. So, when Elizabeth reached the end of her life, Protestant James VI of Scotland was a much more appealing candidate than his mother had ever been, and he became James I of England (which put England and Scotland on the road to becoming one country).