William Bonney was a pioneer with his mother, with whom he was very close. The two of them settled in Colorado and she married a cruel and cold man, who hated William. She passed away of illness not long after when he was barely 14, and William ran away from home, rather than stay with his evil stepfather. He travelled to New Mexico found employment with a man named John Tunstall, an English merchant, whose business sense had made him a rich and influential man. Tunstall took William under his wing.
William worked for him happily, and Tunstall outfitted the boy with his famous pistol and his horse. But Tunstall's success and ethnicity earned him enemies with Irish rival merchants and one day while he, William and and a few other men were riding horseback, a bunch of desperados hired by those merchant rivals ambushed Tunstall's party with a hail of surprise gunfire. Everyone fled, but evidence was shown to suggest they shot Tunstall off his horse, followed him as he crawled away, and shot him again at point-blank range, in a targeted execution.
The party returned after realizing Tunstall hadn't followed, and discovered his body. William, having just lost the closest he ever had to a father figure in his life, vowed revenge. He knew just who had done it, and together with the rest of Tunstall's remaining men, formed a gang called "The Regulators" and began his campaign of murder, eventually spurring the Lincoln County War, earning the name Billy the Kid. During his short life he is reputed to have killed 20 people. He was captured and jailed several times but always escaped- in once instance, a judge said he'd see him "hang, 'til he was dead, dead, dead." and Billy quipped back "you can go to hell, hell, hell."
His ruthlessness was legendary. He killed, forced others to watch, and set bodies on fire to send a message. He used his youthful, innocent appearance to his advantage. He knowingly framed innocent people as his accomplices to have them captured in his stead. The extent of his crimes was not known until after his death, and he was purported to be dubious friends with New Mexico Governor Lew Wallace, to whom he routinely snitched and bartered information to, in return for him to look the other way.
His death was an accident, while trespassing one night he accidentally came face to face with the Governor Wallace in the darkness, who shot him in the chest. People were suspicious of how convenient it was, and to this day people wonder if Wallace had actually helped Billy the Kid fake his death... especially as after, Governor Wallace wrote a biography about his life. To this day, unsubstantiated rumours about people thought to have been Bonney still survive. He was barely 22 years old, that last was heard of him.