In his sermons he denounced heresy and also those Catholics who professed the Faith by words, but acted contrary to it in deeds. Crowds came to meet him and followed him; conversions were numerous,[2] including many Cathars who returned to the Catholic church.
Because of this, a group of Milanese Cathars conspired to kill him. They hired an assassin, Carino of Balsamo. Carino's accomplice was Manfredo Clitoro of Giussano. On April 6, 1252, when Peter was returning from Como to Milan, the two assassins followed Peter to a lonely spot near Barlassina, and there killed him and mortally wounded his companion, a fellow friar named Domenico.[3]