He was killed by stabbing with swords. Henry II spouted off at dinner to be rid of Thomas, and his advisors acted on it.
The Archbishop of Cantebury, Thomas Becket, was murdered December 29th, 1170 by the knights Reginald FitzUrse, William de Tracey, Hugh de Morville, and Richard le Breton. The knights took it upon themselves when they overheard their king, Henry II, asking if no one would rid him of this troublesome priest.
The King of England, Henry II, found Becket troublesome because they had a falling out over how the church punished offenders. Henry II wanted the royal court to have more power and say regarding the punishment. When Becket refused to follow Henry II's orders he fled abroad. Becket returned after approximately six years, but they soon started disagreeing again. That is when the knights overheard King Henry II and decided on their own to travel to Canterbury and kill Becket.
After Becket died, the place where he was killed became a pilgrimage and the pope made him a saint. Henry II asked for forgiveness and went to pray at the spot Becket was killed. While praying, he was whipped by monks for Becket's death.