The central chapters of this book focus on the development and growth of insular origin legends over time by studying a key subset of themes that came to take on particular significance within this corpus. Tracing the expansion and increasing centrality of these themes over time allows us to witness the influence that individual texts within the corpus of material containing early insular origin legends had on the development of these legends themselves. Chapter Three focuses on kin-slaying, first tracing the influence of the biblical legend of Cain and the classical legend of Romulus and Remus on early medieval authors before examining contemporary evidence for kin-slaying in early medieval legal and historical texts. As insular origin narratives expanded, their authors recognised the narrative need to explain ancestral exile. The idea that a foundational ancestor had committed the crime of kin-slaying was introduced via the Brutus story in the ninth-century Historia Brittonum and proved subsequently popular, as exile was theoretically the legal sentence for this crime. As a narrative motif, kin-slaying allowed a people to retain the prestige of ties to ancient dynasties while embracing political independence in the present moment.