DEDHAM -- While most people have an opinion on the death penalty, few have ever witnessed an execution in person.

Yesterday at the Noble and Greenough School, Sister Helen Prejean of the St. Joseph order described her own experience getting to know and then watching convicted murderer and rapist Elmo Patrick Sonnier being killed by electric chair in a Louisiana prison.

Sonnier was executed about 15 years ago by three jolts of 1,900 volts of electricity. Prejean, who met the man after contacting him to be a pen pal, said that after he was declared dead she left the room and vomited.

Hundreds of students packed the school's auditorium to hear Prejean's tale, which she later made into a book, "Dead Man Walking." In 1996, a movie debuted based on the book and starring Susan Sarandon and Sean Penn. Sarandon won the Best Actress Award at the 1996 Academy Awards for her role depicting Prejean.

At the Dedham school's assembly yesterday, Prejean described not only the feelings of Sonnier as he sat waiting for death in his cell, but also those of the shaken prison guards who killed him, and the heartbroken family members whose loved ones were murdered by him.

She said she understood all sides, but Prejean also told the group she personally thinks the death penalty is "torture" and should be abolished.

"We have prisons. We have ways of incapacitating dangerous people," Prejean said.

She cited statistics to back her anti-death penalty stance. She said 117 people on death row have been taken off after being proven innocent. Some recent cases in Illinois, where Northwestern University journalism students proved death row inmates innocent, led the state to place a moratorium on the death penalty.

"Because the system is so flawed, because people are not always well represented (in court), the truth doesn't always come out at trials," Prejean said.

Her latest book about that topic, "The Death of Innocents: An Eyewitness Account of Wrongful Executions," will be published in January.

But even when men are proven guilty without a doubt, Prejean said she believes the death penalty is wrong. She said one of the prison guards who electrocuted Sonnier later quit his job, unable to sleep or eat.

"We've got to ask ourselves, 'If I believe (death row inmates) ought to die, could I do it?'" Prejean said.

In all, Prejean has accompanied five men to their prison executions.

After her discussion, students were allowed to meet Prejean. The author, a member of Amnesty International, also set out a petition for students to sign calling for a national moratorium on the death penalty.

The connection between Prejean and Nobles began in June 2004 when she was part of a group that included several Nobles faculty and students in Billings, Mont. The group was there to construct a sweat lodge for a Cheyenne tribe.