Dr. Martin MacNeill, a 57-year-old Utah doctor, is on trial for "intentionally overdosing" his wife after she had plastic surgery, ABC reports.
Court proceedings over the 2007 death of Michele MacNeill begin just a few months after the defendant was released from federal prison, where he served four years for identity theft.
The woman's body was found in the bathtub at the couple's home days after the surgery, which her husband had insisted she get.
MacNeill used his wife's recovery period to "obtain the necessary drugs and set in motion the circumstances to … cause Michele's death," court documents allege.
Prosecutors say MacNeill killed his wife because she had confronted him about his affair with a woman named Gypsy Willis. The doctor hired Willis to be a live-in nanny two weeks after his wife's death. The MacNeills had eight children.
Willis testified on Wednesday that she met MacNeill online in 2005, and that their sexual relationship began a year later.
In 2009, both MacNeill and Willis were convicted of fraud. They had stolen the identity of one of Dr. MacNeill's three daughters in order to make it appear that Willis and Dr. MacNeill were married, according to the Daily Herald.
Prosecutors say people close to MacNeill portray him as someone who frequently spoke about killing.
Anna Osborne, another woman with whom he reportedly had an affair, said the doctor told her he had tried to kill his mother, discussed murdering his brother in a bathtub, and had offered to kill Osborne's husband in order to free her from an abusive relationship, according to an affidavit.
Osborne also reported that MacNeill had discussed the various ways one could kill someone and get away with it, prosecutors say.
One of MacNeill's daughters, whose name has not been released, told the Associated Press that he had once threatened to kill her in order to end her drug addiction.
Defense attorney Susanne Gustin says her client's past does not make him a killer.
"He's done some bad things in life," she told the court, "but does that mean he's a murderer? No."
Alexis Somers, one of the doctor's daughters, disagrees. "I knew from the day my mom died that my father killed her," she told the Deseret News.
After her mother died, Somers won custody of her three youngest siblings when MacNeill planned to give them away.