The concept of vassalage and how the system of feudalism worked. Depending on the power of the Monarchy in any given nation varied. In some nations the monarchy may have been so limited that the king had no army of their own and relied on the private army of each of their nobles. In other nations even if the king had their own army, it generally wasn't greater then the combined forces of their vassals. By simply murdering a noble, a king would have violated their own oath of reciprocal protection, and proven to their other nobles that they to were in danger. There are cases of rulers slandering there own nobles or placing there loyalties in doubt as an excuse to destroy them with the tacit approval of their other vassals. The Spanish inquisition was on occasion used in this manner, if a noble were to become a problem, simply accuse them of being a "secret jew/muslim". They would be found guilty after tortured confessions and then the crown would split their wealth and land with the inquisition. There is also highly suspicious motives behind the destruction of the Templar Knights. Rumors were spread by the French crown, a false pope was used to remove their church sanction, and there confessions were tortured. Coincidently, the French king was able to avoid paying his massive debts to the order with their destruction, enrich himself by claiming their property, and was very upset that they managed to avoid the capture of the majority of their monetary wealth.

Primarily though, a ruler who killed there nobles without the appearance of wrongdoing on the nobles part, had a strong chance of finding themselves at war with their own vassals. Even killing a foreign noble might find the noble relatives of the victim leading armies against them. The final consideration would be finding themselves unable to take loans because they were known for not only failure to pay the debt (bad enough) but killing the debt holder, who would make a loan with that expected outcome?

On the other hand, killing a usurer (jewish moneylender/wealthy merchant) could have little consequences if undertaken without fanfare.

Contrary to popular misconception, medieval kings were not absolutist rulers who could arbitrarily imprison or execute subjects who fell out of line. Indeed, until quite late in the medieval period, political executions were very rare. For example, William the Conqueror (duke of Normandy from 1047 and king of England from 1066 to 1087) could certainly be very brutal, reflected in his laying waste to the North of England after the region rebelled in 1069 and the extremely harsh punishments he prescribed to poachers caught hunting in his royal forests, which normally included blinding or other forms of mutilation, both of which caused deep and long-standing resentment among the native English (cf the "Rhyme of King William" in the Peterborough Version of the Anglo-Saxon Chronicle). Yet barely ever did he execute any leading political opponents. For example, after he defeated his cousin Guy of Brionne at the battle of Val-es-Dunes in 1047, which ended the civil war in which the two of them fought for control of the duchy of Normandy, William actually let Guy go free. Then again in 1075, when William the Conqueror faced an armed revolt from three of his earls, Earl Roger of Hereford, Earl Ralph of East Anglia and Earl Waltheof of Northampton (the last surviving native English earl), who were trying to overthrow him, most likely to install King Sweyn II of Denmark in his place, William had Roger punished with imprisonment in Normandy, Ralph punished with exile in Brittany and Waltheof with imprisonment in the Tower of London followed by execution the following year. Waltheof's execution was the only political execution in William the Conqueror's entire reign, it hugely shocked contemporaries with some chroniclers, such as Orderic Vitalis, portraying Waltheof as almost something of a martyred saint, and historians to this day are still puzzled as to why William the Conqueror did it. Throughout the next couple of centuries, political executions would remain extremely rare in England. This was because kings were, like their nobles, bound by the culture of chivalry, one of whose key tenets was mercy, especially towards fellow members of the warrior elite. It was only in the late medieval period (14th and 15th centuries) that political executions became more common, as enshrined in things like the English Treason Act of 1351 (which for the first time in English law defined treason, which could mean plotting to kill the king, raping the king's wife or daughter, leading an armed rebellion against the king, counterfeiting the national coinage or murdering high ranking royal officials, and made it a capital offence), but by that point legal and judicial processes had developed to the point that the king could not execute people on a whim, especially nobles who had secured the right to be tried by their peers in the House of Lords. This may all seem quite tangential, but its important to understand.

As for financial demands, quite a few medieval English kings did impose quite onerous demands on their baronial subjects. The most infamous example is King John (reigned 1199 - 1216), who engaged in what appeared to most people at the time like outright robbery against his barons. For example, Nicholas de Stuteville was charged £6000 (this would have been the equivalent of hundreds of millions of pounds/ dollars today) to inherit his estates when really by feudal custom he should have only charged £150. This was what led to the English barons revolting and forcing John to sign the Magna Carta in 1215, which after it was reissued twice in the reign of John's son and successor, Henry III, in 1220 and 1225, secured the English nobility various rights to protect them against arbitrary financial burdens from the crown. By the time of John's grandson, Edward I, English kings had largely shifted away from these traditional feudal payments to instead relying on national taxation, agreed upon by parliament, and loans from Italian merchant banks to fund their military ventures, yet as part of political construction after the 15th century civil wars, Henry VII (reigned 1485 - 1509) used bonds and recogniscances, written agreements in which one agreed to pay a large sum of money if one failed to show sufficient loyalty to the king. These were incredibly unpopular, and one of the first moves of Henry VII's son, Henry VIII, when he became King in 1509 was to have the ministers entrusted with levying and collecting these bonds and recogniscences tried, arrested and executed on trumped up charges.