Complex sentence: Hungarian tribal society was not in favor of primogeniture, but of agnatic seniority as order of succession, which made other males of the Árpád dynasty, cadet lines, dangerous to the incumbent king.
Split into simpler sentences: Hungarian tribal society was not in favor of primogeniture, favoring instead agnatic seniority for determining the order of succession. This made other males of the Árpád dynasty's cadet lines dangerous to the incumbent king.