A jury rejected an insanity defense and found prison inmate Joseph Druce guilty Wednesday of first-degree murder in the strangulation of pedophile priest John Geoghan, a central figure in Boston's clergy sex abuse scandal.

Druce admitted sneaking into Geoghan's prison cell in August 2003. He jammed the door shut with a book, then beat and strangled the 68-year-old before guards could stop him.

The defense had argued that Druce was mentally ill and under the delusion that God had chosen him to kill Geoghan and send a message to pedophiles around the world.

Prosecutor Lawrence Murphy presented another picture of Druce, describing him as a conniving killer who planned the murder for weeks so he could be a "big shot" in prison.

"He was not a mentally ill person, raging out of control," Murphy said. "He's a calculating individual who waited for his opportunity."

The jury of five women and seven men deliberated for about six hours before reaching the guilty verdict.

Druce, 40, is already serving a life sentence for killing a man who allegedly made a sexual pass at him after picking Druce up hitchhiking. He unsuccessfully used an insanity defense during that 1989 trial.

With the new conviction, he'll face another life sentence without the possibility of parole.

On the witness stand, Druce described a troubled childhood in which his father beat him and his mother. He said he was physically and sexually abused at a residential school for troubled children.

Druce said he killed Geoghan to avenge the innocent children the defrocked priest was accused of molesting. He said he overheard Geoghan advise other inmates on how to molest children and say he planned to move to South America after prison so he could resume working with children.

"I had seen myself as the designated individual who had to put a stop to the pedophilia in the church," Druce said.

Geoghan was in prison for fondling a 10-year-old boy, but he was accused in lawsuits of sexually abusing some 150 children.

His case helped spark the clergy sex abuse scandal that spread worldwide after church personnel records revealed that the Boston Archdiocese had transferred Geoghan from parish to parish, even after allegations of abuse surfaced.