Nepocide

Morvryn […was] reclaimed by the gentle care of his twin-sister, Gwendydd, the mother of the ill-fated youth whom he had deprived of life. […] Jeoffrey of Monmouth composed a Latin poem, dedicated to Robert, Bishop of Lincoln, in which he describes with considerable spirit, though in rugged verse, the insanity of the involuntary nepocide: […]

Source: wiktionary

I could not but be comforted by his support, not alone for the sake of my grandfather, whom I'd long since forgiven his attempted nepocide and wished only well, but also because, as Bray's subsequent speech indicated, he had apparently more confidence in me than I'd had for the last quarter-hour in myself.

Source: wiktionary

In between royal hopefuls and the crown lay many hazards, including not only infant mortality but martial defeat and/or exile, as well as (sometimes suspicious and always deadly) accident, (probable) fratricide and nepocide, and even regicide: three of the four deposed and murdered English kings (Edward II, Richard II and Henry VI) belong to this period.

Source: wiktionary