To reach the Roman church of S. Pietro in Vincoli from Via Cavour during my Roman sojourn, I climbed a stone stairway, officially called Via S. Francesco di Paola. But that stairway is also known as Salita dei Borgia, because it passes under an archway which was part of the house of Vannozza Cattanei, mistress of Pope Alexander VI and mother of Giovanni, Cesare, Lucrezia, and Gioffre Borgia. Giovanni Borgia, Duke of Gandia (referred to by his Spanish name, Juan, in the TV series), may well have been murdered there in 1497, possibly by either one of his two brothers.
That the just concluded Showtime series The Borgias unambiguously portrays Cesare Borgia as his brother's killer is just one of the liberties that series may have taken with history. For example, the SHO series portrays Cesare Borgia as killing three Sforzas - Giovanni (Lucrezia's first husband), Ludovico (Duke of Milan), and Benito (Catarina Sforza's son). In fact, all three outlived Cesare. Savanarola was executed in Florence, not Rome. Nor was the real Michelleto illiterate. Cesare likely did kill Lucrezia's second husband, Alfonso of Aragon, but most likely for political not personal reasons (after which, the historical Lucrezia went on to a happier and more successful third marriage).
Of course, The Borgias is hardly the first historical drama to be historically challengeable, none of which necessarily detracts from its dramatic artistry. And the series certainly does an effective job of dramatizing the milieu of Renaissance Italy - its politics, diplomacy, warfare, and court life - opening a window onto a world we assume to be so very different from our own. In the process, however, it may narrow our ability to understand and appreciate the real historical Borgias and their actual world.
Thus, presumably to highlight Cesare and make him more sympathetic to the TV audience, the older Borgia brother (the Duke of Gandia) is portrayed the most malevolently. In fact, however, he did not kill the Ottoman Prince Cem, for example. And, whatever his faults, his daughter became a nun and his grandson a Jesuit and a saint - Saint Francis Borgia. (Juan still has descendants with the Borgia name in South America, one of whom became President of Ecuador!)
Nor does the series do full justice to the complex pontificate of Alexander VI, although all the Borgias, Alexander among them, do tend to be portrayed less uni-dimensionally and with more nuance than Juan. Historically, Alexander seems to have been an effective administrator and reasonably successful politically in his Italian renaissance context, something that does actually come across in the series, despite the emphatic overlay of scandal that seems everywhere to surround him.
Apparently, Cesare Borgia was thought of as an irreligious man even in his own time, which is certainly saying something. So it is not necessarily anachronistic for the TV series to portray him that way. But the complex emotional relationship depicted between Cesare and his father may in the end be beyond our ability to access with any accuracy. It may make sense for our psychologized culture to project onto father and son such intense sentiments as were expressed in the dramatic confrontation between them in the next-to-last episode. But we really cannot access their feelings, nor would they have shared our modern psychologized vocabulary in which to express them.
That, of course, is always a problem with any modern historical drama. We today are much more interested in the feelings and emotional lives of historical characters than their contemporaries probably were, and we have invented a complex vocabulary to talk about such things, a vocabulary that they and their contemporaries lacked. Similarly, a secularizing society such as ours, even when it admits the factual omnipresence of religion in previous periods of history, will increasingly find it hard to comprehend the complex way in which religious belief and religious feeling were interwoven into all aspects of ordinary life.