Royals and nobles killing their relatives over titles was extremely common. Just for example:

• Yaropolk I of Kiev ordered the assassination of his brother, Oleg. The third brother, Vladimir the Great, fled to Scandinavia and then returned with an army and had Yaropolk killed.
• Ioannes I Tzimiskes assassinated his maternal uncle, Nikephoros II Phokas, with the help of his uncle's wife and some disgruntled generals.
• Fatimid caliph Al-Hakim bi-Amr Allah is suspected to have been assassinated on the orders of his half-sister, Sitt Al-Mulk.
• Fernando I of Castile killed his brother-in-law, Bermudo III of León, in battle and then claimed the Leonese throne in right of his wife.
• Sancho II of Castile was probably assassinated on the orders of his siblings, Urraca of Zamora and Alfonso VI of León.
• Boleslaus I, Duke of Bohemia has been long-suspected of ordering the murder of his older brother, Wenceslaus (the subject of the carol, "Good King Wenceslaus".)
• Andronikos I Komnenos ordered the murders of his cousin's two children, Alexios II Komnenos and Maria Porphyrogennete Komnene (and a lot of others) to make himself emperor.
• István IV of Hungary was poisoned by supporters of his nephew while besieging Zemun.
• Arthur of Bretagne was likely killed on the orders of his paternal uncle, John I of England.
• Albrecht I of Germany was assassinated by his nephew, Johann Parricida, who was aggrieved that his uncle had not given him any land.
• Edward II of England ordered the execution of his cousin and enemy, Thomas of Lancaster. He was later deposed by his wife and disgruntled barons and probably secretly killed in prison.
• Joanna I of Naples was deposed and later murdered by her second cousin and nephew by marriage, Charles of Durazzo. Charles then tried to claim the Hungarian throne from another cousin, Maria of Hungary, but was assassinated on the orders of her mother, Elizabeta Kotromanić.
• Pedro I of Castile was personally murdered by his half-brother and enemy, Enrique de Trastámara.
• Richard II of England was deposed by his cousin, Henry Bolingbroke, and then likely killed later in prison on Henry's orders.
• Henry VI of England was deposed by his distant cousin, Edward IV, and eventually murdered in prison.