Throughout history, all across the world, cats have been viewed in both a positive and negative light. The one cat that has seen the most negative connotations connected to it is the black cat, and no man was as adamantly against such creatures as Pope Gregory IX.
Pope Gregory IX, a zealous proponent of Papal supremacy, is well known for his efforts to eradicate heresy and heretics (basically non-Catholics). Instituting the Papal Inquisition as a means of bringing order and legality to the process of dealing with heresy, Pope Gregory did his best to stop heresy from spreading without having to use torture as a tool of investigation or as a punishment.
One of the various heresies he condemned was a German heresy known as Luciferian; it was a form of devil worship. Pope Gregory condemned this heresy by issuing the papal bull, Vox in Rama. The bull details the initiation rites of the sect, which includes kissing the buttocks of a statue of a black cat that comes alive after the initiation meal and walks backward with its tail erect towards the new initiates. The initiation meeting ends with a wild or*y, which ends with a brief conversation between the cat and the cult members.
After condemning this practice through his bull, Pope Gregory called the religious and secular authorities in the Church to take action against the cult's participants, which included the elimination of black cats.
However, it turns out that these instruments of evil that Pope Gregory viewer as a representation of the Devil were actually protectors of humans. A few decades after Pope Gregory issued the papal bull that essentially condemned black cats and called for their elimination, Europe was plagued by the Black Death. Brought over from Asia, the Black Plague was carried by rats and infected hundreds of Europeans, bringing them to an untimely death. It is well known that cats are rats' natural predator, so it wouldn't be that far off to assume that the decimation of the cat population led to the boon of the rat population. Basically, by calling for the death of cats, Pope Gregory set hundreds of Europeans up for death by having one of the various limiters of the Black Plague be eliminated.