Lazarus is a former Los Angeles Police Department detective accused of killing Sherri Rasmussen in 1986.
Struggling to contain his tears, John Ruetten described the day he came home to find his wife's lifeless body on the floor. Then he went on to describe his long sexual relationship with Lazarus.
"We were just friends," Ruetten testified. "I never thought she was involved."
Prosecutors say Lazarus was the jilted lover who sought revenge against Rasmussen.
On the stand, Ruetten said he never considered Lazarus his girlfriend, even though they had a sexual relationship that lasted several years after they met at UCLA in the early 1980s.
The relationship continued even after Ruetten was engaged to Rasmussen in 1985.
Ruetten and Rasmussen were still newlyweds when the 29-year-old nursing supervisor was bludgeoned and fatally shot in her condo in February 1986.
"This is a really emotional day," said John Taylor, the attorney for the Rasmussen's family. "The testimony takes the family back to a really special, happy time in their lives, which very quickly gets very, very ugly."
Some of the Rasmussen family members broke down and cried during much of the testimony.
"To go back and feel that again, it's just been a real emotional roller coaster," Taylor said.
Jurors also heard from Lazarus' former roommate, retired LAPD Officer Mike Hargreaves, who said he remembers a night when Lazarus was upset, complaining that Ruetten had broken up with her.
Lazarus has pleaded not guilty. She was arrested in 2009 after investigators linked her DNA to a bite mark found on the victim's arm.
If convicted, Lazarus faces up to life in prison.
Testimony continues for the next several weeks.