No matter how hard a period drama tries to maintain historically accurate, there will always be a few things about the show that aren't true to history. In order to amp up the drama, many historical TV shows develop their villains into truly monstrous characters, even if, historically speaking, the person the character was based on was actually a pretty nice guy. Here are some examples of shows where the villains are way meaner than they ever were in real life.
One of the most loathsome characters on the show is Giovanni Sforza, Lucrezia's ex-husband. Blinded by his prejudice against her family and pressured into the marriage by his own clan, Giovanni took out his anger on Lucrezia. His beastly bedroom attacks on her are likely the root of her twisted plans to lash out at anyone who hurts her now (we're looking at you, Juan).
But in real life, Giovanni Sforza actually didn't touch her for months after they were married, as he was respectful of her young age and childlike innocence. The wedding contract specified that Lucrezia wouldn't leave Rome for a year, and Giovanni wasn't even present at the ceremony; they were married by proxy.
This HBO series was critically acclaimed for its nuanced characters and sumptuous production values. But seductive, manipulative Atia of the Julii isn't exactly grounded in reality. She's a conglomerate of the historical women Clodia and Atia Balba Caesonia. The latter woman was written about by Roman historian Tacitus, who described her as highly moral: a far cry from the scheming, cruel woman we see in "Rome."
No one would argue that Jack the Ripper wasn't a terrible killer, but in this sci-fi series, he's also a time traveler who has the ability to teleport. And his killer tendencies are actually caused by an evil energy creature.
But that's nothing compared to the way Nikola Tesla gets treated. In real life, he was a Serbian inventor. In the show, he's a vampire with magnetic and electrokinetic powers.
Generally speaking, most Robin Hood adaptations portray Prince John as far more evil than he was in real life. Historically, he was highly educated (he wrote several books and was an esteemed legal mind of the time), and he's considered to be the founder of the British Navy. But time after time, poor Prince John gets portrayed as everything from stupid to usurping to cruel, with one scene in "The New Adventures of Robin Hood" showing him murdering a ton of peasants in order to appease the gods.
As an interesting side note, one "Doctor Who" serial presents the theory that Prince John's "evil deeds" were done by an android impersonating the Prince. It may not be historically accurate, but the Doctor's tirade to his companions about how the lives of good men can be twisted by history is worth remembering.