Stephen Alexander Marshall was born on August 9, 1985, in Fort Worth, Texas. He was the son of Ralph Arthur Marshall and Margaret Malia Diekelmann. When he was a child, his family moved to Cape Breton, Nova Scotia. In 1996, his parents went through a divorce, affecting his early life.

In 1999, Stephen moved to Culdesac, Idaho, with his father, Ralph. Ralph served three years as the mayor of Culdesac. Stephen had some trouble during his teenage years. In April 2001, when he was 15 years old, he was charged with aggravated assault. This incident happened after he brought an AR-15 rifle onto his lawn where two young people were fighting.

After the events in Idaho, Ralph moved away to live in Arizona and then Maine. Meanwhile, Stephen returned to live with his mother in Cape Breton during the summer of 2003. He wanted to join the Army but was rejected due to his asthma.

On April 13, 2006, Stephen Marshall traveled from North Sydney, Nova Scotia, to Houlton, Maine, to visit his father. During his journey, his car broke down in Sackville, New Brunswick, and he called his father for help. On April 15, Marshall took his father's Toyota pickup truck and three firearms without permission. He drove towards LaGrange, Maine, looking for registered sex offenders.

Early in the morning on April 16, at around 3 a.m., Marshall shot and killed 57-year-old Joseph Gray through the front window of his home in Milo, Maine. Gray was asleep in his living room at the time. A few hours later, at approximately 8:15 a.m., Marshall arrived at the mobile home of 24-year-old William Elliott in Corinth. After Elliott opened the door, Marshall shot him multiple times, killing him, before fleeing the scene. Elliott's girlfriend managed to record the license plate number of Marshall's father's truck.

Marshall then drove to Bangor, Maine, where he abandoned the truck and one of the rifles. He boarded a bus headed for Boston, Massachusetts, at 1:45 p.m. On the bus, law enforcement was alerted about his possible presence due to his earlier actions. The bus broke down, and while waiting for a replacement, police stopped the bus to apprehend him. Before he could be taken into custody, Marshall shot himself in the head, dying later that night in a Boston hospital.

After his death, investigators examined his laptop and discovered that he had visited the homes of other registered sex offenders. Marshall killed Joseph Gray and William Elliott, who were both on the sex offender registry.