The two youngest Moran children didn't really know much of their mother, because she died early on in their lifetimes. Sylvester was the only one of them to actually remember her, because he had been ten when she was killed. He and his father had agreed not to tell the younger boys about their mother being murdered, instead, the story was that she died in a car accident, which, in fact, she had. But they didn't go into details about why the accident happened. She was killed to get to their father. His employers had her tailed and killed in the accident.
Sylvester moved out when Sebastian was ten, Cyrus being five. Augustus then took Sebastian under his wing and taught him his way around guns. His father taught him how to protect himself and his brother, while also teaching him skills such as fixing cars, skinning animals for their pelts, and hunting in general, which Sebastian would later use as his living.
When Sebastian was around 15, his father's employers kidnapped his family. They were brought to an unmarked warehouse, blindfolded and gagged. Sylvester had also joined in the same business as his father when he moved away. Their captors took Sebastian and Cyrus' blindfolds as they interrogated Augustus and Sylvester. When they refused to give the information they were being asked for, their captors killed the men in front of Sebastian and Cyrus, and Sebastian, never being without a weapon of some sort, went on a blind rampage, his vision going red as he slaughtered the men. He released Cyrus and they took their murdered family and burned their bodies.
Sebastian didn't speak for months on end. He took care of Cyrus, cooking for him, caring for him when he was sick, and hunting periodically. One day, after a hunt, Sebastian came home to find Cyrus missing. He panicked at first, when he did not find his little brother, thinking perhaps there had been men from his father's employers that had found and kidnapped him. That is, until he found the note. His brother had left home, hoping to leave Sebastian to enjoy his life his own way. Without the burden of having to care for his younger brother.
Sebastian didn't stay in the empty house long after his brother's departure. He moved to Asia a few weeks later, not bothering to take anything but a few guns and knives; just weapons. He hunted just about anything, but his specialty was tigers. He loved the skins of the tigers, either keeping them or selling them off to buy ammo and new guns. Soon, he heard about a war that was raging and he decided to sign up for the military, hoping to get more experience with guns and such in doing so.
So he went to war. There he met Dr. John Hamish Watson, a rather attractive man, whom Sebastian came to fancy. During a late part of the war, their camp was attacked and to protect the defenseless Dr. Watson, Sebastian shot anything with a gun pointed at him, not even realizing he was also shooting his own men. Though he saved John's life, he was dishonorably discharged from the military.
For about a year or so after being discharged, he had gone through several odd jobs, nothing sticking. He was thinking about giving up, when he was contacted by a man named Jim Moriarty. He started working for this man as his bodyguard and personal sniper, ridding the world of any who he saw fit.