While we still have a massive fascination with the idea of royalty today, it's important to remember that a system of succession made for some pretty horrific rulers put in charge of millions of people. In particular, these eight rulers proved that you not only didn't have to be qualified to become royalty, but you didn't even have to be sane.
Justin II (Byzantine Empire)
The successor to Justinian, who is often considered the most beloved Emperor in the history of the Byzantine Empire, Justin II had some pretty big shoes to fill. Unfortunately, he was far from qualified for the task; not only did he lose much of the empire's territory to the Persians, but historians report that he heard voices in his head, and would often scream and hide under his bed to escape them.
He also essentially had the mind of a child, and would have to be pushed around a wheeled throne to stay calm. Otherwise, he would fly into fits and bit the heads of his servants. Seriously.
Charles VI (France)
You don't get a moniker like "The Mad" for nothing, and Charles VI more than earned that title during his reign. He infamously attacked some of his own knights while in pursuit of an assassin, would run though the castle howling like a wolf, and was afraid to let people touch him because he was convinced he was made of glass. These days, most people suspect he had a form of schizophrenia.
King Farouk (Egypt)
The last ruling king of Egypt, Farouk was essentially the reason the people of Egypt decided to do away with a monarchy entirely. During his reign, he grew to over 300 pounds by allegedly drinking 30 bottles of soda a day and eating caviar straight from the can. He also allegedly became a total kleptomaniac, infamously stealing a pocket watch from Sir Winston Churchill. Yes, THAT guy.
That's just the start though. After having a nightmare about lions coming to eat him, he had all the lions in the Cairo Zoo shot. Finally, and most egregiously of all, he invited the Nazis to invade Egypt during the Second World War, as he was unhappy with British rule. The people were less than enthusiastic, and overthrew Farouk in 1952.
Carlos II (Spain)
A direct result of the massive amounts of inbreeding going on in Spain's royal family, Carlos II was severely disfigured from birth; with a huge elongated head, a misshapen body and a jaw that could not close so he could eat. He was also ignored by teachers and relatives, meaning he was still mentally a child when he took the throne (which happened due to literally all of his relatives dying off).