It is all about understanding de jure and de facto rule. De jure rule refers to the "Legitimate" and "official" ruler. De facto means the guy actually calling the shots. Most of the times when regicide occurs, the usurper is already the de facto ruler. The murder isn't the point where power shifts; it's the last event in a series of coups, replacements and schemes to finalize and officiate the change. It's the reason why most regicides seem awkwardly simple / easy: that's because there wasn't really anyone opposing it. There's been lots of examples brought up above, but the most important one's been missed: the assassination of Tsar Paul I. of Russia. The man was excessively unpopular with Russian nobility and the key power holders in his Empire - he granted freedom to serfs, limited the nobility and went as far as to UNCOVER CORRUPTION in the treasury! Also, he was pro-Napoleon. Can't have that, right? Pretty much the majority of Russian leadership at the time agreed he's better off gone. They simply entered his bedchamber unopposed, gave him an abdication declaration to sign and when he refused to, they strangled him to death and told his son "You are Tsar now." The son didn't punish any of them, in fact, one of the assassins - General Bennigsen - became Russia's field marshal later. Russia was nowhere near centralized and nobility held a lot of sway and power. Unlike most western European regicides, they didn't even care about the publicity much - it went all pretty open, because the majority of nobles simply agreed with what they were doing. In fact, the majority of the regicides were done/by/ the guards and followers, who could get close enough to land the murder. Consider two things: 1. Why would people accept him as a king? 2. Why wouldn't people accept another character as a king instead? (Reasoning they are the TRUE HEIR!) An easy way to handle this would be for your character to make contact with anti-war nobility and build up a formidable opposition faction, then approach the enemy the kingdom is at war with and agree to a premature peace deal in turn for non-interference (or even better, concede to the enemy some lands from the nobles that didn't join his faction). After that, the board is pretty much set and your character can walk up to the king and strangle them - since all those who could act are in favour, can do it in broad daylight.
Correct. Pardon the omission. It doesn't appear that any of the current royals anywhere near the top of the line of succession are or are likely to be Roman Catholic, but you are correct. That is the rule, and has been for a couple or three hundred years or so.
Let's not forget that someone who kills a king may not wish to become king him or herself. Take the case of the regicides of Charles I of England in 1649. Charles I was not a popular monarch. I'm fuzzy on the details, but he was involved in and was considered to have been responsible for not one but two civil wars, as well as possible treason against his own country. Eventually, Parliament empaneled a court of high justices to conduct a trial of the king. Charles I was convicted of treason and was executed by beheading. This was the start of the period when England was governed by Oliver Cromwell. After Cromwell's death, however, the monarchy was restored and those who had participated in the trial and execution of Charles I found themselves declared to be regicides, and they were hunted down. I grew up in a small town just outside of New Haven, Connecticut. New Haven has three major streets named Whalley Avenue, Dixwell Avenue, and Goffe Street. These streets were named after three of the judges who had served on the tribunal that sentenced Charles I to death. Those three escaped to New England and hid out in New haven and the surrounding countryside. There is a high cliff on the west side of New Haven (cleverly named West Rock), at the top of which is a big pile of rocks known as the Judges' cave. It marks one of the locations where the three regicides are believed to have lived for a while during their flight from arrest and prosecution/persecution (depending on where your sympathies lie). https://en.wikipedia.org/wiki/List_of_regicides_of_Charles_I
I've been doing a lot of reading on the Old Testament of the Bible, where regicide is pretty common. In most cases, the usurper had rounded up an army of supporters and followers who allowed him to seize the throne and execute the king. And he made sure he had at least a small contingent of supporters in important places already.
The US Secret Service, which protects the president, are (supposedly) loyal to the office rather than the person. That was the original concept behind the Praetorian Guard in ancient Rome; they eventually became kingmakers (well, emperor-makers) themselves and took an active role in politics. If the regicide is "public" then there is the possibility of civil war. IMHO, everything is going to hinge on the intrigue behind the scenes. Who is in on it, and why? Does the prince have allies amongst the nobles and the military? How much influence do the "commons" have? The commons were mostly illiterate peasants concerned with day to day survival. Well, of course, the nobility was against him; he was taking away their power and privilege, completely overturning the status quo. Indira Gandhi was assassinated by her own security detail. Again, the machinations behind the scenes and public perception will have a huge impact how the king's death and the legitimacy of the heir are perceived.
