>>213014815Because murder only became an issue once civilizations rose. City-state leaders didn't want their subjects being killed by their other subjects, they needed them for taxes and warfare with other city-states. Before then tribal warfare was a simple part of life, albeit not a day to day one. But you would need to kill enemy warriors to protect your tribe and prevent women from betraying your tribe.
Incest was a far more dire threat because tribes were already related to an extent. Cousincest was normal in tribes (look at that Harvard study from a decade ago showing third cousin incest being extremely fertile pairings). Close kin inbreeding, however, would unleash those negative recessive genes much more easily.
tl;dr Murder as a taboo is a result of civilization, sibling and parent incest as a taboo is more ancient but cousin incest was fairly common until recently.