Who was responsible for Thomas Becket's death in 1170 and why?
Beckett and the king (Henry II) were increasingly in conflict over the jurisdiction of secular courts over the clergy as part of Henry's attempts to loosen the power of Rome over England via the Church, this eventually led to Beckett being exiled to Normandy but then re-integrated with the agreement between Rome and Henry reached in the Constitutions of Clarendon, which Beckett (archbishop of Canterbury) refused to sign
Things came to a head when the Heir to the Throne was ordained in York thus usurping the Canterbury as the primary diocese in England, Beckett excommunicated the three bishops who performed the ordination.
This led the King to make the oft quoted "who will rid me of this turbulent priest" - probably as a turn of phrase, but it was taken as a real question by four knight who rode to Canterbury and slew Beckett
The knights were Reginald FitzUrse, Hugh de Morville, William de Tracy and Richard le Breton