Thomas Brennan, a youth gymnastics coach, confronted Nassar in a Michigan courtroom on the second day of a sentencing hearing for the former doctor, who pleaded guilty in November to multiple counts of sexual assault.
"For the record, go to hell," Brennan said, glowering at Nassar.
Brennan's outburst came as he stood next to one of Nassar's victims, Gwen Anderson, as she recounted the molestation she suffered at the hands of Nassar.
At one point during Anderson's tearful testimony, Brennan barked at Nassar, "look at her."
Nassar has sat through the proceedings with his head bowed, not making eye contact with the victims. He apologised for his crimes at the November hearing, local media reported.
Brennan told Ingham County Court Judge Rosemarie Aquilina that he sent more than 100 girls to Nassar, whom he once considered a mentor.
"The guilt I feel for that is hard to fathom," he said.
More than 100 victims are providing victim-impact statements during the four-day hearing, prosecutors said.
The judge has addressed each victim and repeatedly said she will make certain Nassar gets a lengthy prison sentence. Prosecutors are seeking a life sentence.
"He will die there," Aquilina told one victim on Wednesday. "The next judge he faces will be God." Aquilina is set to impose sentence on Friday.
Nassar is already serving a 60-year prison after pleading guilty in July to child pornography charges in federal court.
A mother of a one-time U.S. Gymnastics team member, Maggie Nichols, who tried to qualify to compete in the 2016 Olympic Games, also criticized Nasser on Wednesday.
"A real doctor never sees a child alone," said Gina Nichols, who said she is a registered nurse and whose husband is a doctor. "You're a serial child molester - a paedophile."
Nichols also criticized USA Gymnastics for not properly vetting Nassar, who served as the team's physician through four Olympic Games.
Olympic gold medallists Aly Raisman, Gabby Douglas, McKayla Maroney and Simone Biles have gone public in recent months, saying they were assaulted by Nassar while undergoing treatment.
(Additional reporting and writing by Keith Coffman in Denver)
