Ruth Pelke spent her final moments on the floor of her home in Gary, Ind., bleeding to death from 33 stab wounds. She was 78. Her assailants, a group of teenage girls playing hooky from school, had not broken into her home. Pelke welcomed them in that day — May 14, 1985 — when they knocked on her door asking to attend the Bible lessons she had taught for decades.

Inside the house, according to accounts of the crime, the girls struck Pelke over the head with a vase. When she fell to the ground, Paula Cooper, then 15 years old, began bludgeoning her with a butcher knife. The girls stole $10 and drove off with Pelke’s Plymouth. They were arrested shortly thereafter, having bragged about the act.

The brutality of Pelke’s murder brought national attention to Gary, an industrial city in the throes of painful decay. Even greater attention came in 1986 when Cooper, after pleading guilty to the killing, was sentenced to death. The New York Times reported that, at 16, she was the youngest woman in nearly 100 years to receive a capital sentence in the United States. Three accomplices received prison sentences.

Bill Pelke, a crane operator at Bethlehem Steel and a veteran of the Vietnam War, was devastated by the murder of his grandmother. Already struggling amid bankruptcy and the break up of his marriage, he boiled in anger at God for having allowed him to survive Vietnam, only to then endure such grief.

Mr. Pelke was initially satisfied by Cooper’s death sentence, he said, but underwent a transformative experience at work, ensconced in a steel mill crane, one November day the year after his grandmother’s death. Devoutly Christian, he began praying as tears streamed down his face. In his mind, he said, he saw a photograph of his grandmother smiling serenely — a portrait such as one that might appear in a church directory — but with “tears flowing out of her eyes and rolling down her cheeks.”

“At first I thought they might be tears of pain,” he later wrote in a memoir, “but I immediately realized that they were tears of love and compassion for Paula Cooper and her family.”

In her abiding religious faith, he concluded, his grandmother would not have wanted Cooper to be executed. She would have wanted her killer to be forgiven. “I felt Nana wanted someone in our family to have that same love and compassion,” Mr. Pelke wrote. “I felt the responsibility fell on me.”

Mr. Pelke, who forgave and befriended his grandmother’s murderer, helped lead a successful effort for her death sentence to be reduced to a prison sentence, and became a nationally known advocate for the abolition of the death penalty, died Nov. 12 at his home in Anchorage. He was 73. The cause was a heart attack, said his partner, Kathy Harris.

In 1993, Mr. Pelke co-founded Journey of Hope — From Violence to Healing, an advocacy group led by family members of murder victims seeking to end the death penalty. Often traveling in a dilapidated bus, Mr. Pelke and other activists crisscrossed the country speaking in churches, schools and other gathering places and protesting outside prisons and the U.S. Supreme Court.

“The death penalty has absolutely nothing to do with the healing the murder victim’s family members need when a loved one has been killed,” he said in 2009. “It just continues the cycle of violence and creates more murder victim family members.”

Along with other activists, Mr. Pelke was credited with helping change public opinion in the United States about the death penalty. The Pew Research Center has documented a marked decrease in support among Americans for capital punishment since the 1970s, although a survey in 2018 showed a slight uptick in support.

“When you look at the whole sweep of it, it was amazing what it did, because it had such an effect on people, and in so many states,” Helen Prejean, a Catholic nun and anti-death penalty activist whose story was dramatized in the 1995 film “Dead Man Walking,” said in an interview about Mr. Pelke’s work. He “was the anchor of the organization,” she said. “He was the heart.”

In his speeches, Mr. Pelke would often display a photograph of his grandmother. But he also emphasized the humanity of her killer, who had endured a scarringly abusive upbringing. He corresponded with Cooper for years, visited her repeatedly in prison and sought a relationship with her family as he campaigned — over the objection of some of his relatives — against her execution.

When he met Cooper the first time, he told the Associated Press, “I gave her a hug . . . I stepped back. I looked her in the eyes and I told her that I loved her and had forgiven her.”

Because of her youth, Cooper’s case attracted international outcry, including an appeal for clemency from Pope John Paul II. At the time of her sentencing, Indiana law allowed the execution of defendants as young as 10. In 1989, amid a series of changes in state and federal laws increasing the minimum age for execution, the Indiana Supreme Court reduced Cooper’s death sentence to 60 years in prison.

(In 2005, the U.S. Supreme Court ruled that the execution of a defendant who was younger than 18 at the time of the crime violated constitutional protections against cruel and unusual punishment.)

Mr. Pelke welcomed Cooper’s release from prison in 2013, when she was 43 years old. She had earned a bachelor’s degree during her incarceration and, after her release, found work and became engaged. But in 2015, she died by suicide. “I have taken a life and never felt worthy,” she wrote in a letter to her fiance, according to the AP.

Reached by phone, Rhonda LaBroi, Cooper’s older sister, said that when she first heard of Mr. Pelke’s effort on behalf of her imprisoned sister, she could not bring herself to believe the reports. The friendship that she ultimately formed with Mr. Pelke, she said, was “out of nowhere.”

“He was just beautiful,” she said in an interview. “From the first day that he decided to forgive to the last day of his breath”

William Robert Pelke was born in Lebanon, Ind., on Sept. 16, 1947. His father was a steelworker and his mother was a homemaker.

Mr. Pelke began working for Bethlehem Steel before his service in Vietnam, for which he was awarded a Purple Heart, and returned to his work there after his discharge. He received a bachelor’s degree in pastoral theology from Hyles-Anderson College, a Bible college in Crown Point, Ind., in 1977.

His marriages to Mary Lohman and Judy Falls ended in divorce. In 1999 he moved to Alaska to be with his partner, an anti-death penalty activist. His 2003 memoir, like his organization, is titled “Journey of Hope . . . From Violence to Healing.”

Besides Harris, of Anchorage, survivors include three children from his first marriage, Christina Pelke of Portage, Ind., Robert Pelke of South Haven, Ind., and Rebecca Pelke Bigsbee of Greenwood, Ind.; two stepchildren from his second marriage, Ken Michiaels of Union Mills, Ind., and Taniya Michiaels of Wanatah, Ind.; a sister; and numerous grandchildren and great-grandchildren. A stepson from his second marriage, Jami Michiaels, died in 2005.

David Kaczynski, who helped authorities identify his brother, Ted Kaczynski, as the Unabomber in the mid-1990s, today chairs the board of Mr. Pelke’s organization. He credited Mr. Pelke with helping families of crime victims and families of perpetrators bridge a “psychological divide” that “we can only cross through forgiveness.”

“Even though our hearts were aching for the victims, we had no way to express it,” David Kaczynski said in an interview, recalling his own family’s experience. “To see that healing was possible, that we could find common ground, and to see that embodied in Bill Pelke’s journey . . . was deeply meaningful to me.”

“This was Bill’s mission,” he added, “asking us all to open our hearts [and] turn our personal pain into compassion for other people’s suffering.”

Mr. Pelke, for his part, said that “forgiving Paula Cooper did more for me than it did for her.”

“It gave me the philosophy of life,” he told the Anchorage Daily News in 2004, to forgive “a neighbor who complains about the noise, a driver who cuts you off. It really is a wonderful way to live.”