Pope Sixtus IV

Pope Sixtus IV (Latin: Xystus IV; 21 July 1414 – 12 August 1484), born Francesco della Rovere, was Pope from 9 August 1471 to his death in 1484.
Sexius introduced the period of unabashed, unconcealed, relentless pursuit of personal gain and power politics. While his cultural concerns were admirable, his feuds, conducting wars on Venice and Ferrara and his feud with the Colonna family.  (Barbara Tuchman, The Pursuit of Folly)
His accomplishments as pope included building the Sistine Chapel; the group of artists that he brought together introduced the Early Renaissance into Rome with the first masterpieces of the city's new artistic age. He also established the Vatican Archives. Sixtus furthered the agenda of the Spanish Inquisition and annulled the decrees of the Council of Constance. He was famed for his nepotism and was personally involved in the infamous Pazzi Conspiracy (Conspiracy to overthrow the Medici) .
His involvement with the failed Pazzi conspiracy was undeniable but his response to the failure was to excommunicate Lorenzo de Medici and the city of Florence.
Sextus ignored calls for reform, eventually imprisoning and starving to death Archbishop Zamonetic who had called for a council to attend to church reforms.
Nepotism:
Sixtus IV sought to strengthen his position by surrounding himself with relatives and friends. In the fresco by Melozzo da Forlì he is accompanied by his Della Rovere and Riario nephews, not all of whom were made cardinals. His nephew Pietro Riario also benefited from his nepotism. Pietro became one of the richest men in Rome and was entrusted with Pope Sixtus' foreign policy. However, Pietro died prematurely in 1474, and his role passed to Giuliano della Rovere.
The secular fortunes of the Della Rovere family began when Sixtus invested his nephew Giovanni with the lordship of Senigallia and arranged his marriage to the daughter of Federico III da Montefeltro, duke of Urbino; from this union came a line of Della Rovere dukes of Urbino that lasted until the line expired in 1631.[ Six of the thirty-four cardinals that he created were his nephews.
According to the later published chronicle of the Italian historian Stefano Infessura, "Diary of the City of Rome", Sixtus was a "lover of boys and sodomites" - awarding benefices and bishoprics in return for sexual favours, and nominating a number of young men as cardinals; some of whom were celebrated for their good looks.[10][11][12] However, Infessura had partisan allegiances to the Colonna and so is not considered to be always reliable or impartial.
The English churchman and protestant polemicist John Bale writing a century later, attributed to Sixtus "the authorisation to practice sodomy during periods of warm weather" to the "Cardinal of Santa Lucia". However, such accusations are easily dismissed as anti-Catholic propaganda.
Slavery
The two papal bulls issued by Pope Nicholas V, Dum Diversas of 1452 and Romanus Pontifex of 1455, had effectively given the Portuguese the rights to acquire slaves along the African coast by force or trade. These concessions were confirmed by Sixtus in his own bull, Aeterni regis of 21 June 1481. Arguably the "ideology of conquest" expounded in these texts became the means by which commerce and conversion were facilitated
In November 1476 Isabel and Fernando ordered an investigation into rights of conquest in the Canary Islands, and in the spring of 1478 they sent Juan Rejon with sixty soldiers and thirty cavalry to the Grand Canary, where the natives retreated inland. Sixtus' earlier threats to excommunicate all captains or pirates who enslaved Christians in the bull Regimini Gregis of 1476 could have been intended to emphasise the need to convert the natives of the Canary Islands and Guinea and establish a clear difference in status between those who had converted and those who resisted.