The consensus on the Kennedy assasination already exists. See this answer by /u/thistorian.

We aren't exactly sure why Philip II was killed. If you're unfamiliar, he united the Macedonians and created a very powerful army that his son, Alexander, used to conquer the known world.

It was widely believed in the 13th century that Arthur of Brittany — King John's nephew and rival claimant to the English throne after the death of Richard I — was murdered while held in captivity in the castle at Rouen, either by William de Braose, one of John's court favorites, or by John himself.

You can probably guess which version of the story was more popular amongst the English aristocracy, who pretty much universally despised John.

As for Arthur himself, he's simply not mentioned in any surviving historical records from after his incarceration in 1203. It's still not known what actually happened to him.

The death of King Ludwig II of Bavaria in June of 1886 was certainly suspicious, and has been the source of controversy almost from the time of the event to the present.

Ludwig is probably best known as the builder of Castle Neuschwanstein, the famous castle that served as inspiration for Disney's Cinderella's Palace and the backdrop of many a postcard and the occasional motion picture.

Ludwig was known to be erratic and impractical, and became more so over time, giving speculation to some form of mental disorder; insanity was known to run in his family. He was not engaged in running the government, withdrawn from his ministers and the world in general, and perhaps most importantly, he had a penchant for building very expensive and impractical fantasy-castles like Neuschwanstein.

Days before his death, he was declared legally insane and incapable of reigning. Of course, speculation revolves around whether or not this was politically motivated opportunism by Ludwig's enemies in the government, taking advantage of a ruler's eccentricities.

His body was found floating in a Bavarian lake, along with psychiatrist Dr. Gudden, the pair had gone for an evening stroll. There were signs of a struggle found on the lake shore. The doctor had water in his lungs and therefore is believed to have drowned, although he also had a head wound that may have indicated he was knocked unconscious. The autopsy of Ludwig was very badly done and inconclusive. There were no witnesses to the event.

Speculation runs that Ludwig was murdered by members of his government (or by persons acting on their behalf), or that Ludwig committed suicide and possibly Dr. Gudden's demise was in the course of trying to prevent it, or of course that their deaths were pure misadventure. The official verdict from the inquiry into his death was Ludwig committed suicide.

Author Peter Glowasz makes a case that Ludwig was murdered. (http://www.koenig-ludwig.org/index.php?option=com_content&view=article&id=164:death-by-the-lake-&catid=49:articles&Itemid=67). This site, mostly in German, does contain a reasonable selection of English articles.

This essay does a good job of providing a survey of the topic, indicating the ambiguity and controversy surrounding the topic: http://skeptoid.com/episodes/4407

Here is the small municipality of Schwangau in Bavaria's tourist site on Ludwig, his life, his castles, and his demise: http://schwangau.de/his-mysterious-death-646.html

The suspicious death under confinement of the Hapsburg King Philip II's eldest son, Carlos, in 1568 became the subject of a number of conspiracy theories. Philip forbade the court to speak of his son's imprisonment, leading to rumors that Carlos was arranging to meet with Dutch rebels or that he was even having an affair with the Queen. Although Carlos was mentally unstable by most accounts, the later Dutch rebels would assert that his father killed his heir because Carlos was more favorable to the Dutch. Orangist propaganda made much of of the rumors that Philip murdered his son and asserted the untimely deaths of Philip's wife Isabella de Valois was foul play. William the Silent's Apology claimed Philip was "an incestuous king, who slew his own son and murdered his wife."

As for consensus, most historians of Spain feel that Carlos's death was the result of medical issues from a trepanning procedure or some other ailment associated with his confinement. In addition to Philip II's general probity, murdering his only heir would throw Spain into a dynastic succession crisis.

Sources

Arnade, Peter J. Beggars, Iconoclasts, and Civic Patriots: The Political Culture of the Dutch Revolt. Ithaca: Cornell University Press, 2008.

Parker, Geoffrey. Philip II. Boston: Little, Brown, 1978.