Tsarevich Ivan Ivanovich
Ivan the Terrible never had good impulse control. One night in November 1582, the Russian Czar Ivan IV was busy yelling at his son Ivan Ivanovich's pregnant wife. The argument became violent, and when the czar flew into a rage and struck her, Ivan Ivanovich rushed to intervene. Ivan IV lashed out at his son, striking him in the head with a scepter. There's a reason he's called Ivan the Terrible.
The young heir to the throne survived for a few days before finally dying of his wounds, but the fallout from his death lasted much longer. Ivan Ivanovich was the only one of Ivan the Terrible's sons who stood a chance of running the country -- his younger brother Feodor was sickly and mentally disabled. When Feodor did, in fact, ascend the throne after his father's death, he was essentially a figurehead controlled by ministers.
Feodor never produced an heir. After he died, conditions in Russia, which had steady declined during his reign, finally flipped over and burst into flames. The years after his death are known as the Time of Troubles, an unprecedented era of anarchy and chaos. Over the two decades that followed, a third of the population, about 5 million people, either starved to death or were killed in the civil strife of the newly lawless Russia [source: White].