“Some sixty years later the dispute between king and Church flared up again. This time it took the form of a quarrel between Henry II (1154-1189) [of England] and Thomas Becket [...] Just as in the case of the emperor Henry IV [of the Holy Roman Empire] and Pope Gregory VII, personalities played their part in the quarrel. To Henry II Becket, his former chancellor and his own choice as archbishop, seemed to be breaking his obligations, both of personal and of feudal loyalty. For Becket the need to live down his past as a wordly courtier and to prove his true commitment as a churchman seems to have been almost as important as the issues in the dispute. A series of compromises failed to resolve the conflict, Finally, Becket excommunicated several English bishops who had supported the king and Henry showed his anger in public. Four knights took it as an invitation for action and murdered th archbishop in Canterbury cathedral, on 29 December 1170.

The details of the murder have come to us from a letter by Becket’s secretary and friend, the historian John of Salisbury.
[...]
As a sign of Becket’s holiness, John mentioned especially that, after his death, he was found to have worn a hairshirt ‘crawling with lice and worms’. It was to be a long time yet before personal cleanliness was held to be a virtue next to godliness.”

— H.G. Koeningsberger,
Medieval Europe 400-1500, pp. 171f.