Jack Levin, a criminologist who is co-director of the Brudnick Center on Violence and Conflict at Northeastern University, said it was likely that Mr. Moody possessed a common trait among killers.

“Serial killers often attempt to justify their killing sprees, and they usually do it by dehumanizing their victims,” Dr. Levin said.

But the case in South Carolina is otherwise odd, he said.

“It’s very unusual that someone would set out to kill large numbers of sex offenders.”

Mr. Moody is not the first person accused of targeting sex offenders. As recently as last month, a California jury convicted a 36-year-old man of killing a neighbor who was a sex offender, and a Washington State man was sentenced in 2012 to life in prison for a pair of similar killings.