Queen Elizabeth's grandfather, King George V, apparently died due to mercy killing. A new documentary, George V: The Tyrant King, details the contents of his doctor's private diaries.

King George V's physician Lord Dawson of Penn allegedly wrote in his diary of euthanizing the King at 11:00 p.m. on Jan. 20, 1936 so that his death would be printed in the front pages of newspapers. The physician revealed giving the King a deadly dose of cocaine and morphine to ease his pain. 

The doctor detailed that King George V badly suffered from his condition in the "last stages," which could last for "many hours." So, a decision was made to inject the monarch at the right timing so that his death will make it to the press.  

According to Daily Mail, King George V's real cause of death was not known to the public for many decades until the discovery of the diary in 1986. Scholars argued whether this was mercy killing or murder since no one had the absolute authority to take the King's life.  

King George V suffered from many health conditions. He had recurring respiratory problems, including chronic bronchitis. He, also, had a life-threatening condition called septicaemia and in the last two years of his reign, the King had to relegate some of his duties to his first-born son, David, who will later be crowned King Edward VIII but would abdicate from his throne. 

Royal expert Ingrid Stewart said that on his final days, King George V suffered from colds but was bedridden. He, apparently, knew his time was coming but scholars at that time argued he could not have agreed to euthanasia because his wife, Queen Mary of Teck, was a highly religious royal. 

Then, Princess Elizabeth was only 10 years old when King George V passed away. At that time, she was still unaware of her challenging future. Her Uncle David was supposed to be the next in line to throne but he decided to give this up for his wife, the American divorcee, Wallis Simpson. 

After David or King Edward VIII abdicated, Prince Albert, then the Duke of York, became King George VI, making his daughter Princess Elizabeth the heir to the throne. King George VI reigned from 1936 until his death in 1947, when Her Majesty was in her mid-20s and a young mother to Prince Charles and Princess Anne, as well as a wife to her cousin, Prince Philip. 

Prince Philip and Queen Elizabeth are direct descendants of Queen Victoria, who had nine children. She was the mother of King George V and Princess Alice, who is the maternal great-grandmother of Prince Philip. According to the Independent, Prince Philip and Queen Elizabeth are third cousins

In those days, members of the royal family were encouraged to marry each other because they could lose their right to the throne if they ended up with a commoner. In modern times, especially in Queen Elizabeth's reign, the rules of royal marriage have obviously changed.