Inside the Mind of an Alleged Brazilian Serial Killer

— -- An accused serial killer from Brazil pleaded guilty to killing 39 people -- including many women and homeless people.

Tiago Rocha, 26, was arrested Thursday after police in Goiania linked a gun found in his home to a string of women's murders earlier this year, the Associated Press reported. He confessed to at least 39 killings in midwest Brazil dating back to early 2013, including several young women and homeless people, according to the AP.

But his attorney Thiago Vidal told the Folha de S.Pauo newspaper that his client confessed to crimes he did not commit because of aggressive questioning.

Experts say his kill list is a common one.

"There's a type of serial killer called the mission serial killer and they think they're doing some good deed for society -- homeless people would fall in line with that," criminologist Scott Bonn told ABC News today. He cited Itzcoatl Ocampo as an example, former Marine who returned home to Los Angeles in 2011 on a mission to rid the city of homeless people.

"When he was caught, he rationalized this as, I was doing something good for society," Bonn said. "So there's a precedence for this sort of behavior."

If convicted, Rocha would be one of the deadliest serial killers in modern history, said Bonn, author of "Why We Love Serial Killers."

"He's up there," he said. "One of the more prolific [serial killers] in our country is Ted Bundy, who killed 36."

Criminology professor Steve Egger said serial killers are often motivated by troubled childhoods.

"There's not one formula. Frequently, it's rage built up because of the way they were treated as children and they want to get back at society," he said. "And one way to get back at society is killing. Or, people they're killing may be symbols of people who beat them, and sexually abused them as children."

Like Rocha, serial killers often appear to be normal.

"They look like anybody else," Egger said. "That's how they get away with it."