Butler’s Book of Saints gives a very laudatory account of this Dominican:
In 1205 the glorious martyr Peter was born at Verona of heretical parents. He went to a Catholic school, and his Manichean uncle asked what he learnt. “The Creed,” answered Peter; “I believe in God, Creator of heaven and earth.” No persuasion could shake his faith, and at fifteen he received the habit from St. Dominic himself at Bologna. After ordination, he preached to the heretics of Lombardy, and converted multitudes. St. Peter was constantly obliged to dispute with heretics, and although he was able to confound them, still the devil took occasion thence to tempt him once against faith. Instantly he had recourse to prayer before an image of Our Lady, and heard a voice saying to him the words of Jesus Christ in the Gospel, “I have prayed for thee, Peter, that thy faith may not fail; and thou shalt confirm thy brethren in it.” Once when exhorting a vast crowd under the burning sun, the heretics defied him to procure shade. He prayed, and a cloud overshadowed the audience.
In spite of his sanctity, he was foully slandered and even punished for immorality. He submitted humbly, but complained in prayer to Jesus crucified. The crucifix spoke, “And I, Peter, what did I do?” Every day, as he elevated at Mass the precious blood, he prayed, “Grant, Lord, that I may die for Thee, Who for me didst die.” His prayer was answered. The heretics, confounded by him, sought his life. Two of them attacked him as he was returning to Milan, and struck his head with an axe. St. Peter fell, commended himself to God, dipped his finger in his own blood, and wrote on the ground, “I believe in God, Creator of heaven and earth.” They then stabbed him in the side, and he received his crown.
Despite Butler’s assertion, the heretics in question were not, strictly speaking, Manicheans, but rather Catharites (aka Albigensians), members of a dualist sect with deep support in northern Italy and southern France. They appeared to the casual observer to be Christians but rejected the God of the Old Testament, believing that material Creation was evil. Their goal was to escape the world of flesh and be reincarnated — thus they rejected eating meat and having sex. They were pacifists at first, but persecution led them first to targeted assassination of priests and then to outright rebellion.
Peter of Verona was an Inquisitor (in fact the Chief Inquisitor for northern Italy) and thus a suitable victim for Catharite assassins. He became known as Peter the Martyr and the name “Pietro Martire” became a popular one for boys in the late Medieval period.