I do not know about any executions. However monarchs held alot of power until feudalism came about really. Feudalism created many independent polities. Essentially the king held less and less power as more and more Lords with knights, forts, and peasants/serfs came about. It wasnt instantaneous as different countries held on to kingships power more than others. In particular England did descent job of holding its monarch mainly in power. As compared to france were the king for a very long time essentially held no power due to feudal fragmentation. Each lord had their own court, a seigneurial court (made up of knights/nobles) while the kings had their high courts which were made up of the lords). In the middle east, particularily crusader states, kings began to lose their power when even in the kings court or high court the kings power was cut back due to previous power abuses. In jerusalem kings could not even remove land from someone without having a day in court and not only the person with the land would be on trial but also the king as well, being judged by the lords of the land. In england after 1215 with the signing of the magna carta, kings power was also restricted and kings had to answer to the people and courts and follow the laws, not just the subjects. So after these events occured the King would be held to follow the laws just like the people. So if the monarch broke the laws he would be punished accordingly.