The word bastard used to simply mean "born out of wedlock", but came to mean a ruthless person. It featured in South Park a lot in it's second meaning: "Oh my God! They killed Kenny!" "You bastards!"
So how did the word Bastard get it's second meaning?
Because of William the Bastard
Also known as William the Conqueror, the man who invaded Britain in 1066, and whose descendants still sit on the throne, nearly a thousand years later.
The above image depicts William as depicted in the Bayeux tapestry, during the Battle of Hastings, lifting his helmet to show he was still alive. Source: Wikimedia commons
William was the illegitimate son of Robert Duke of Normandy, also known as Robert the Magnificent, and his mistress Herleva the tanners daughter. But because Robert was unmarried, and delighted with his little son, he left the Dukedom to him, and got all his nobles to swear fealty to the seven year old boy before embarking on the Crusades, from which he never returned.
Naturally Robert's other relatives thought the dukedom should have gone to the more legitimate members of the family, and William's childhood was plagued with assassination attempts and four of his guardians were killed in succession. But instead of traumatising him, it simply made him tougher. By the time he was an adult and exercising full control over his dukedom, he had turned into a very formidable opponent. But he continued to sign his name William Bastardis - William the Bastard.
He defeated the forces trying to invade his duchy (including an invasion attempt by the King of France), and then took territory from his neighbours. In one siege, the inhabitants hung tanning leathers from the ramparts of the walls, shouting "hides for the tanners son", to mock his mother's lowly birth. The Bastard coldly ordered the entire town burnt to the ground. No-one was spared, not even the hermit who had lived alone in a cell in the cathedral. After that display of ruthlessness, no-one gave him any trouble any more.
Invasion of England, 1066
William had been promised the throne by Edward the Confessor of England, who died without children, and who was a distant relative of William's on his father's side. But on Edward's death, the throne had been seized by a lord in Edward's court, Harold Godwinson, who had himself crowned King. Harold knew that William would contest this, and so planned a defence force of 10,000 men. But someone else was eyeing England - the formidable Viking, Harold Hardrada, King of Norway. He launched an invasion, landing in the north of England, near York. Harold Godwinson, hearing the news, marched north, getting to York, 190 miles away, in just five days (note that in the 18th century that journey would have taken a week with a coach and horses so Harold's men must have ridden like the wind). He surprised his enemy and defeated and killed the King of Norway and then marched south again, to await William the Bastard.
William landed in England on 14th October 1066, at Hastings, and a battle was fought. At first it seemed that Harold's forces had the upper hand, they controlled a hill from which they could throw spears at the Normans. But they were also tired from their march north and back, and not as disciplined as William's men. At one point, William's men feigned a retreat, and the Saxons, thinking they were winning, broke their ranks to run after them, only to find themselves surrounded and slaughtered.
The Bastard becomes King
Of course winning a battle isn't the end of things. A King has to consolidate his power, and put down rebellions. He lay siege to towns in the south that defied him, starving them into submission. And when the north of England rebelled, he embarked on what is known euphemistically as the "Harrying of the North". This involved burning villages to the ground, killing everything including livestock so that fugitives couldn't re-group and feed themselves, and killing anyone who gave them trouble. England's population was just 1 million at the time, and 100,000 lost their lives in the Harrying, on top of those who had lost their lives in the southern sieges. A loss of over 10% of the population counts as a genocide.
The Bastard had never lost a battle in his entire career and prevailed in putting in place a new feudal system over the English. He went into the history books as William the Conqueror. But his other name - The Bastard - lives on when people wish to swear at their enemies.