Hanover, N.H. ? The bond between the teen-agers accused of killing two Dartmouth College professors may have sparked an “evil game” of dare that led them to commit increasingly serious crimes, a criminologist says.

Robert Tulloch and James Parker grew up in tiny Chelsea, Vt., playing soccer, rafting rivers and climbing mountains together. While they had other friends, those who knew them said the two did everything together.

Tulloch and Parker

“The evil part of this I believe is the relationship between these two boys,” said Jack Levin, a Northeastern University criminologist. “I don’t think either one of them would have done this by himself. It was the chemistry between them and the bond they shared.”

According to a new indictment released last week, prosecutors believe robbery was the motive for the slaying of Half and Susanne Zantop, whose slashed and stabbed bodies were found in their home in January 2001.

“This is a crime that was senseless, and that’s why so many people have trouble understanding it,” Levin said.

Tulloch, 18, is charged with first-degree murder, which carries an automatic life sentence without parole.

He has indicated that he will use an insanity defense at his trial, scheduled to start in April, and his lawyer plans to challenge the accuracy of forensic evidence collected in the case.

Parker, 17, pleaded guilty to reduced charges and agreed to testify against Tulloch.

Two or three years ago, Tulloch and Parker broke into at least two houses when no one was home, in one case making themselves something to eat, according to friends and several Chelsea, Vt., residents.

“That was only the rehearsal,” Levin said. “They got their feet wet in burglarizing empty houses. They honed their skills. They practiced for the big one.”

In June 2000, the two upped the stakes, deciding to kill homeowners and steal their bank cards, according to the new indictment.

The indictment says the teens tested their plan once that summer and three times in January 2001 at homes where either the residents were not home or wouldn’t let them in.

“These two guys are dabblers,” Levin said. “They aren’t career criminals. They don’t spend 100 percent of the time doing crimes. They treated it as a game. It was an evil game, but it wasn’t like they were preoccupied by it.”

The indictment says the pair talked their way into the Zantops’ home by saying they were doing an environmental survey. After stabbing the couple to death, they drove away and disposed of some evidence.

They allegedly drove back to retrieve knife sheaths they left inside, but changed their minds because of a police car in the driveway.

“My feeling is there was a game of chicken, where they both dared each other to do more and more riskier and difficult kinds of deviant things,” Levin said.

But what allegedly prompted them to graduate from petty burglary to murder?

John Kirkpatrick, a University of New Hampshire criminologist, believes the teens may have planned to use the knives to terrorize rather than kill.

“This has all of the markings of some horrible fantasy gone terribly wrong,” Kirkpatrick said.