CHICAGO (CBS)– Brendt Christensen’s eyes stayed locked on his ex-wife as she testified Friday.

The former University of Illinois student is accused of kidnapping, raping and killing Chinese scholar Yingying Zhang.

READ MORE: Jury Selection To Begin Monday In Jussie Smollett Trial

His defense team called his ex-wife up in an effort to paint Christensen as a man with mental health issues who does not deserve the death penalty.

Prosecutors said he killed Zhang in the apartment the couple shared.

Michelle Zortman described her ex-husband as a man with a bad drinking problem that derailed their marriage.

The day Yingying Zhang went missing, Zortman was out of town on a trip with another man to the Wisconsin Dells.

The same spot where she and Christensen went on their honeymoon.

“He was upset about it,” she testified today.

When she returned home a couple days later, she says Christensen showed her a blood stain on his mattress that she hadn’t seen before.

“He said it was a nose bleed,” she said. Later adding, “It was sizable.”

Prosecutors have not said which stain that is but they have shown pictures of stains as evidence.

But, they do say the FBI found Zhang’s DNA on Christensen’s mattress.

READ MORE: 29-Year-Old Man Shot In Evanston

His ex-wife testified that she saw him leave their apartment that Monday with a huge duffel bag, but it didn’t appear anything was inside.

Prosecutors argue it could have been carrying clothes or other evidence.

Weeks later, Christensen attended a memorial walk for Zhang with his ex-girlfriend.

She was wearing a recording device for the FBI and the night of the walk she recorded him describing in graphic detail how he raped and killed Zhang.

His attorneys are trying to attack the credibility of that recording.

Christensen’s ex-wife testified that she picked him up for a ride that same night and said he was noticeably drunk.

Prosecutors also played a tape of him describing his drinking and depression at University of Illinois counseling session in march 2017, months before Zhang’s disappearance.

He said he was fascinated by serial killers and had thoughts of hurting others but never followed through.

“I’ve always been interested in the bad guys,” he said.

Zortman said things were never the same after Christensen shared his “scary thoughts” with her in late 2016 and she didn’t want to sleep in the same room with him.

MORE NEWS: Car Crashes Into Pond In Aurora

Closing arguments start Monday morning and then the jury will decide whether he is guilty.

Tim McNicholas