There's an old saying that goes, "Absolute power corrupts absolutely." That corruption seems to go right into the brain of some rulers. For some reason, royal families seem to have a higher-than-average incidence of mental illness. From high fantasy to historical dramas, these TV shows feature royal bloodlines with plenty of terrifying, insane rulers.
One of the creepiest, most deranged rulers in "The Borgias" is King Ferdinand of Naples. He is old and almost completely senile when the series begins. However, in his youth, King Ferdinand was feared for his brutal treatment of his enemies. If you displeased Ferdinand, chances are that you would end up killed and then preserved by taxidermy. The bodies of Ferdinand's enemies were then placed in a tableau in a special room where Ferdinand was recreating da Vinci's "Last Supper" with corpses.
In the TV adaptation of the Merlin legend that airs on the SyFy channel, nearly every man in the Pendragon clan has a history of mental illness. King Uther is able to keep his sanity for some time, but eventually goes mad after his illegitimate daughter steals the throne for herself. Poor, crazy Uther had himself a severe mental breakdown.
There are all kinds of crazy nobles in "Game of Thrones," but the Targaryens and the Lannisters are definitely among the most disturbed. Aerys Targaryen, the so-called Mad King, was so insane that his own sworn guard decided to kill him for the good of the Seven Kingdoms. It's explained that the tendency toward madness in these families was likely caused by inbreeding over the generations.
While the English monarchy has had many insane rulers over the years, "The Tudors" didn't have too many over the course of the series. Jane is shown going a bit mad toward the end of her life, and Henry likewise goes a bit cuckoo after Jane's death when designing a new building. While Henry might not be totally crazy, you've got to admit that it does take a bit of a sociopath to kill off multiple wives.