Patricia Catanio told jurors that mental health professionals use a scale of one to 20 to rate the severity of sexual addiction, and that Jason Williford scored an 18.
"It's the highest score I've ever seen," said Catanio.
Williford has pleaded not guilty in Taft's murder. He could face the death penalty if convicted.
Defense attorneys have not disputed that Williford is responsible for the killing, but they maintain he has serious mental health issues and is not guilty of premeditated first-degree murder. Prosecutors must prove premeditation to send Williford to death row.
Police have said Williford attacked Taft - a member of the N.C. State Board of Education - inside a home on Cartier Lane in the late night hours of March 5 or the early morning hours of March 6, 2010. She died at WakeMed a few days later. The home is in the same neighborhood where Williford and his ex-wife Jessica Foote lived.
On the night she was attacked, Taft and her sister were house-sitting while Taft recovered from a surgery. Her sister found her bloodied body. According to testimony from the medical examiner, the state school board member had been sexually assaulted and was killed by a blow to the head.
Williford's friend Daniel Bartose told jurors Thursday he partied with Jason the night Taft was attacked. He said they drank liquor and beer and snorted crushed Ritalin.
Bartose said that at least a week after that night, Williford asked him a strange question.
"He asked me: 'Can God forgive you for whatever you do?' And I thought that was kind of a chilly question," said Bartose.
On the witness stand Wednesday, psychiatrist Philip Hillsman said he treated Williford for mental issues and alcohol addiction, but he wasn't told of Williford's sexual problems.
Williford's mother Pam also testified of a long history of mental health problems within the family. She also said Jason showed problems from an early age. In addition to learning issues such as ADD, she said he had anger problems and had a hard time getting along with other children.
Williford's mother also testified that as he got older, he began to see evidence of sexual disorders which she described in detail for the jury. She also said she saw evidence of drug and alcohol use.
Taft served on the State School Board for 15 years. She also ran unsuccessfully for a state Senate seat in 2008. Prior to that, she served on the Pitt County Board of Education.