Terri Roberts forgave, too, and now she is sharing her experience with others, saying the world needs more stories about the power of forgiveness and the importance of seeking joy through adversity.
"I realized if I didn't forgive him, I would have the same hole in my heart that he had. And a root of bitterness never brings peace to anyone," Roberts said. "We are called to forgive."
Roberts has delivered the message to scores of audiences, from church groups to colleges, and is writing a memoir. She's even considered traveling to speak in Newtown, Conn., where a gunman killed 20 children and six adults at Sandy Hook Elementary School last year. But she is cautious, mindful an appearance there could give offense.
One of her sons is making a documentary — called "Hope" — about her remarkable journey from heartbroken mother to inspirational speaker.
Zachary Roberts originally conceived the film to help his mother. But it's also proving to be cathartic for him.
"It was like a step toward getting this off my shoulders and being able to speak about it," said Roberts, 35, who lives in Sweden. "I have a kid now, and I don't want this to be one of those dark family secrets that nobody talks about. I want to be OK with it, and I want my daughter to be OK with it."
After filming on location in Pennsylvania, Zachary Roberts and the documentary's producers recently released a trailer and have turned to a crowd-funding website to raise money to complete production.
Roberts appears in the trailer and doesn't mince words about the challenge that faced his mother after his 32-year-old brother's rampage: "How does the mother of a mass murderer move forward in life?"
Terri Roberts' path toward healing and reconciliation began, surprisingly enough, that very first afternoon.
Her husband, Chuck, had wiped away so many tears that he'd rubbed his skin raw. The retired police officer hung his head, inconsolable. "I will never face my Amish friends again," he said, over and over.
An Amish neighbor named Henry told him otherwise. "Roberts, we love you. We don't hold anything against you or your son," Terri Roberts recalled Henry saying as he massaged Roberts' slumped shoulders. "We're a forgiving people."
It was an extraordinary gesture, one that gave Terri Roberts her first glimmer of hope. She calls Henry her "angel in black."
That same day, a counselor helped her realize that "we do not need to live in our sorrow." Her son's rampage was one part of his life, a terrible snapshot, the counselor said. Better to focus on all the good years.