Richard I of England, usually portrayed as a smug Sean Connery type, spent most of his life at war. When he wasn't slaughtering heathens, hating Jews and seizing land, he was at war with his brothers and his father. The man had lived to the then-unbelievable age of 42 when a hundred elite assassins sacrificed their lives to mortally wound him in a dazzling swordfight along a sheer cliff face.
No, just kidding. It was one kid with a crossbow and a frying pan.
The young assassin was such a nobody that historians don't even know his name, calling him either Peter, Dudo, John or Bertran (we'll just call him PDJB). The Lionheart came to PDJB's tiny land to suppress a minor revolt, killing the kid's father and brother during the assault. With most of the castle's defenders dead, Richard the Lionheart took a stroll along the base of the walls.
And there, history tells us, the Lionheart rofled. Because at the top of the wall, all alone, was our friend PDJB, batting away arrows with a dented frying pan, and shouting insults at the top of his young lungs. Spotting the king, the boy fired an arrow and missed. Richard, who clearly thought arrow wounds were things that happened to other people, cheered on the defender. The second arrow buried itself in his shoulder. After an unsuccessful surgery made the wound gangrenous, the dying king had PDJB brought to him, and pardoned the boy. Once the king had died, his soldiers skinned PDJB alive. Then, to drive home the point that everyone was just so irritable about the regicide thing, they hung him.
With Richard dead, his brother John became King of England, and began losing territories from Richard's hard-won empire so fast that you have to imagine he was getting candy and handjobs out of it.
Without This Person, We Might Not Have:
The Magna Carta. Though there is still a debate as to whether Richard was even into the ladies, he was still married to one. If he had survived long enough to have a kid, the crown wouldn't have gone to John, who lost so many battles that he got the nickname "Softsword". After a particularly nasty defeat, his Barons forced him to sign a document that essentially made him a king in name only. Add this to the fact that we remember John now as the sissy, whining lion in Disney's Robin Hood, and you have a pretty fair idea of how bad he was for England at the time.