Why did four knights murder Thomas Becket?

The murder of St. Thomas Becket was the culmination of a long-standing and bitter dispute between the archbishop of Canterbury and King Henry II regarding the rights of the Roman Catholic Church and royal authority.12

The tension intensified over the Constitutions of Clarendon, which sought to restrict ecclesiastical privileges and subject "criminous clerks" to secular punishment. Following years of exile, Becket returned to England and further infuriated the king by excommunicating several bishops who had participated in the coronation of Henry’s son, an act that violated the traditional rights of the see of Canterbury.3214

Upon hearing news of these excommunications while in Normandy, an enraged King Henry II uttered words that prompted four knights to travel to England. The knights confronted Becket in Canterbury Cathedral and demanded he absolve the bishops. When he refused, they murdered him, an act that Becket accepted as a defense of the church.21