“... it’s not unknown in European history, either. Richard III had the two young princes killed, for example.”
During modern European history (1800 a.d. to the present) it has been rare to the point of non-existent. The Middle East is a whole other situation.
When a father has the authority under Sharia to kill his wife or children, or the political authorities can mandate the public murders of non-Muslims, how in Hell can anyone expect governments there to be ethical in its treatment of anyone? It is difficult to trust others for any reason in that area due to the existing cultural and/or religous environment. It is one of the reasons businesses aren’t attracted to certain nations in that area, and the result is underemployment in many regions of the Middle East.
Not for nothing is that geopgraphical area known to historians as the “Graveyard of Empires”.
Well, the whole idea of monarchies with multiple claimants pretty much faded out in Europe by that time, but if you allow the murder of one's political rivals after winning out in a power struggle to count, all I can say is go tell it to Trotsky.