Today the Catholic Pope is a generally respected figure, but that hasnt always been the case. Some have been very despicable people, involved in all sorts of nasty situations. Aside from those who were martyred during the earliest decades of Christianity, a number of popes have been murdered by rivals, cardinals, and even supporters.
Pontian (230 - 235): The first pope to resign was also the first pope we can confirm was killed for his beliefs. Earlier popes are listed as having been martyred for their faith, but none of the tales can be substantiated. We know, however, that Pontian was arrested by Roman authorities during the persecutions under emperor Maximinus Thrax and exiled to Sardina, known as the island of death because no one ever came back. As expected, Pontian died of starvation and exposure, but he resigned his office before he left so that there wouldnt be a power vacuum in the church. Technically, then, he wasnt actually pope when he died.
Sixtus II (257 - 258): Sixtus II was another early martyr who died during the persecutions instituted by emperor Valerian. Sixtus had been able to avoid participating in forced pagan ceremonies, but Valerina issued a decree that condemned all Christian priests, bishops and decons to death. Sixtus was captured by soldiers while giving a sermon and perhaps beheaded right there.
Martin I (649 - 653): Martin got off to a bad start by not having his election confirmed by emperor Constans II. He then proceeded to make things worse by convening a synod that condemned the doctrines of Monothelite heretics doctrines adhered to by a number of powerful officials in Constantinople, including Constans himself. The emperor had the pope taken from his sick bed, arrested, and shipped to Constantinople. There Martin was tried for treason, found guilty, and sentenced to death. Rather than kill him outright, Constans had Martin exiled to the Crimea where he died of starvation and exposure. Martin was the last pope killed as a martyr for defending orthodoxy and Christianity.
John VIII (872 - 882): John was paranoid, though perhaps with good reason, and his entire papacy was characterized by various political plots and intrigue. When he feared that people were plotting to overthrow him, he had a number of powerful bishops and other officials excommunicated. This ensured that they moved against him and a relative was convinced to slip poison in his drink. When he didnt die fast enough, members of his own entourage beat him to death.
John XII (955 - 964): Just 18 years old when he was elected pope, John was a notorious womanizer and the papal palace came to be described as a brothel during his reign. It is perhaps fitting that he died of injuries sustained when he was caught in bed by the husband of one of his mistresses. Some legends say that he died of a stroke while in the act.
Benedict VI (973 - 974): Not much is known about Pope Benedict VI except that he came to a violent end. When his protector, Emperor Otto the Great, died, the Roman citizens rebelled against Benedict and he was strangled by a priest on the orders of Crescentius, a brother of the late Pope John XIII and the son of the Theodora. Boniface Franco, a deacon who helped Crescentius, was made pope and called himself Boniface VII. Boniface, however, had to flee Rome because the people were so outraged that a pope had been strangled to death in such a manner.
John XIV (983 - 984): John was chosen by emperor Otto II, without consultation with anyone else, as a replacement for the murdered John XII. This meant that Otto was his only friend or supporter in the world. Otto died not long into John's papacy and this left John all alone. Antipope Boniface, the one who had John XII murdered, moved quickly and had John imprisoned. Reports suggest that he died of starvation after several months in jail.