Pope Sergius III: ordered the murder of another pope and started the "pornocracy"
Sergius III was Pope from 897 to 911, and has been the only pope known to have ordered the murder of another pope and the only known to have fathered an illegitimate son who later became pope; his pontificate has been described as "dismal and disgraceful." The pontificate of Sergius III was remarkable for the rise of what papal historians call a "pornocracy," or rule of the harlots, a reversal of the natural order as they saw it, according to Liber pontificalis and a later chronicler who was also biased against Sergius III. This "pornocracy" was an age with women in power: Theodora, whom Liutprand characterized as a "shameless whore... [who] exercised power on the Roman citizenry like a man" and her daughter Marozia, the mother of Pope John XI (931–935) and reputed to be the mistress of Sergius III.
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Definitively, pope Alexander VI (aka Rodrigo Borgia.)
He was elected pope because of bribery/simony. He was married but, he could however become pope as long as he renounced his wife. He did that... with other women (adultery, incest, rape). He committed murder when his ennemy were getting to him; he would afterward steel the properties of those he murdered.
He was only suspected, mainly because he used his position (and his army) to eliminate any form of accusations.
He is also remembered for the torture of many prisonners.