From King George III of England to King Charles VI of France to Emperor Justin II of the Byzantine Empire to Ivan the Terrible of Russia to Prince Sado of Korea to Emperor Caligula of Rome to...well, you get the idea. Sometimes the insanity resulted in intense socially phobic behavior, with powerful royalty hiding from the world in their own chambers, afraid to interact even with those who were unquestionably their subjects. Sometimes the insanity resulted in horrific abuses of power, sadistic tortures, rapes, and murders of innocent victims.
Possibly the most sadistic of all European royalty was Countess Elizabeth Bathory. Born of incestuous relationships that spanned multiple generations, the Countess was blessed with great intelligence, astounding beauty, and vast wealth. She seems to have been cursed as well. Based on the descriptions of her torture and murder of young female virgins, she seems to have been quite insane. She was reputed to bathe in the blood of her victims in order to keep her skin looking young and smooth. However, she may have also drunk the blood of those same young women and girls.
The compulsion of certain humans to drink the blood of other humans is known as Renfield Syndrome, based on the insect-eating character in Bram Stoker's "Dracula". Renfield was Dracula's mind-slave, and so did not generally have an easy time of things. When he could though, he would slake his compulsive thirst, not with the gin and tonic of the England he left behind, but with human blood. Renfield Syndrome is a real but rare disorder. It is considered to be a severe psychological problem, rather than a physiological need for human blood. This has not been proven though.
While Elizabeth Bathory may have drunk bodies full of blood during her life, she may have been doing it not because she was insane, but because her own physiology demanded it in order to stay alive. Hundreds of years after her reign over Hungary, there was a series of killings in Europe known as the vampire murders. This was due to the observance of the murderer drinking his victim's blood.
One notable malady that seemed to afflict royal families was porphyria. In this disease, handed down through the roots of family trees, the heme part of hemoglobin is not manufactured properly by the human body. This results in unfortunate consequences for the victim of the disease. One of the most noticeable is that the gums recede significantly, which tends to make the incisors look more like fangs than ordinary teeth. Another is that the cuticles recede dramatically as well. The upshot of that is the fingernails taking on the appearance of claws.
Incest, insanity, bloodthirst, claws, fangs. Born of thirsty and tainted blood. And born with the lust to drink of others'. The vampire. A toast to your health.