CHICAGO (CBS) — Brendt Christensen and his father broke down in tears Wednesday as Michael Christensen pleaded with jurors to spare his son’s life for killing Chinese scholar Yingying Zhang, saying “he has too much to offer.”

Michael Christensen took the stand Wednesday at the death penalty phase of his son’s trial.

A jury last month convicted Brendt Christensen of kidnapping and killing Zhang. The same jury is now hearing testimony from the prosecution and defense before deciding whether to sentence him to death or to life in prison.

Michael Christensen said it has been a trying experience thinking about the possibility his son could be put to death by lethal injection.

“A couple days ago, I had a flash of him on the table, getting injected,” he said.

The elder Christensen said he had to stop thinking about it.

“I had to put it away,” he said.

Brendt Christensen. (Courtesy: Macon County Sheriff’s Office)

Brendt Christensen’s defense attorneys have admitted he killed Zhang, but have argued he doesn’t deserve to die for his crime. Both father and son broke down in tears as Michael Christensen testified on Wednesday, apologizing for what his son did.

Michael Christensen testified “I am just so sorry that my son was the cause of the pain” for Zhang’s family.

According to his father, Brendt Christensen’s mother was an alcoholic throughout Brendt’s childhood, often drinking a quart of liquor a day. He also said Brendt tried to commit suicide at the age of 15 by jumping off a deck, and running in front of a moving car.

“He knew he was trying to kill himself, but he could not explain why,” the elder Christensen said.

His father also said Brendt Christensen suffered night terrors and sleep paralysis from childhood to adulthood.

If a jury does rule in favor of the death penalty, a federal judge would still have to agree with that decision before Brendt Christensen could be sentenced to death.

Zhang’s loved ones took the witness stand on Tuesday, giving testimony that brought jurors and even Christensen to tears as they described the pain of losing her.

Yingying Zhang (FBI)

Zhang’s mother, father, and brother all spoke to jurors through a translator.

“How am I supposed to carry on living?” said Lifeng Ye, the mother of Yingying Zhang, whose testimony was on video. “I really don’t know how to carry on.”

Zhang’s mother talked about how she will not be able to see her daughter get married to her boyfriend like she had planned.

“My daughter did not get to wear a wedding dress,” she said. “I really wanted to be a grandma.”

At one point, as Zhang’s mother described her suffering, a juror started to cry and had to momentarily leave the courtroom.

The judge called a recess so she could rejoin the court. After questioning, she was allowed to remain on the jury.

Zhang’s father, Ronggao Zhang, cried when shown a photo of her and him at a train station on her way to the United States.

“My life without her will not be complete,” he testified. Zhang was killed four months before they were to be wed.

Xiaolin Hou, Zhang’s boyfriend, told jurors she meant “everything” to her family.

“They cannot eat, they cannot sleep,” he said.

He said the only hope the family has left in their lives is that Zhang’s body may be found so they can return with it to China for burial.

Federal prosecutors have said Brendt Christensen has not shown true remorse for killing Zhang, and has never revealed what he did with her body.

Illinois abolished the death penalty in 2011, but since Christensen is facing federal charges, the death penalty is still an option.