BRADENTON, Fla. (WFLA) – The Manatee County Sheriff’s Office released new details Monday behind the horrific deaths of an 18-year-old woman and her 8-month-old son.

Larry Bernard Williams, 20, was charged with murder for the deaths of his girlfriend Diamond Shelman and son Jeremiah Shelman at the Avalon Square Apartments on 14th Street West in Bradenton.

Officials say Williams stabbed Diamond on Sunday morning and then slit her throat. He then spread gasoline around the apartment and set it on fire.

The Manatee County Sheriff’s Office said Shelman died from her injuries and Jeremiah died from smoke inhalation.

A firefighter happened to spot the smoke on his way to work. The firefighter also spotted a car driving from the scene which matched the description of a car owned by Williams.

He was arrested at his home without incident.

Williams later admitted to the crimes and said, “The demon that takes him over” stabbed Shelman and burned her apartment.

Diamond Shelman’s mother, Nakiya Smalls, came to the burnt apartment Monday afternoon to remove some belongings.

Since the death of her daughter and grandson, her world has been turned upside down.

“This was senseless, this was a senseless murder. And this is not my story. I’m not supposed to bury my child, she’s supposed to bury me. And this is something that I can’t fix. I can’t fix this,” Smalls said tearfully.

Prosecutors revealed when he was a juvenile, Williams was charged with capital sexual battery.

Smalls tells us Williams and Diamond met while she was in foster care with Williams’ family. But she recently broke up and moved on with her life.

“She was trying to get away, in the end. I will give her that, she was trying to break away and it seemed like he just wasn’t happy,” said Smalls.

She can’t imagine how anyone could be capable of such a brutal and evil crime.

“If you met her, she had the sweetest spirit, nobody would want to hurt her, it would take an animal to do something like this,” Smalls said.

Diamond Shelman was supposed to start college this week. She was going to study to become a pharmacy tech. Instead, her mother is now planning her funeral.

Williams is being held at the Manatee County Jail on no bond.STORIES OTHERS ARE CLICKING ON-