"A shade he showed us on one side alone"

Centaurs punishing violent sinners
Public DomainCentaurs punishing violent sinners - Credit: Gustave Doré

(Canto 12, line 118)

This is Guy de Montfort, Vicar-General in Tuscany for the Sicilian king Charles I of Anjou. His father Simon, for a while the de facto ruler of England, had been defeated and killed by king Edward I. In 1271, to revenge the death of his father, he murdered one of Edward's relatives, Henry, in a church in Viterbo during church service attended by French king Philip III and Charles I of Anjou. The idea that Henry's heart was honoured on the bank of the Thames comes from historian Villani. The murder caused some commotion at the time, incurring public blame not only towards the murderer but also towards the rulers present at the occasion.