The 900s proved to be a particularly dangerous time for the Holy See, and five other alleged papal murders took place in that century alone. According to African Journals Online, Pope Leo V is thought to have been murdered in September 903 after just one month in the position. Pope John X, who was pope from 914 to 928, lived the last year in his life in prison before being smothered by a pillow (though some historians, per The Lives of the Popes in the Early Middle Ages, Volume 4, maintain he actually died from grief).
Although members of the clergy are supposed to practice chastity, Pope John XII was a notorious womanizer, and rumors (via Learn Religions) have claimed that he was killed in bed by an angry cuckolded husband.
According to Britannica, Pope Benedict VI reigned from January 973 to July 974. He was strangled by Boniface VII, who later ascended to the papacy but today is considered an antipope.
Just a decade later, Pope John XIV would be imprisoned by Boniface VII in the Castel Sant'Angelo in Rome before being executed on August 20 (via Executed Today).
