Michael Christensen said that when Brendt was 15 he once jumped 12 feet off a deck, threw himself down some stairs and ran into the street into an oncoming car.
"He didn't know why, but he knew he was trying to kill himself," Michael Christensen said.
The night terrors Brendt experienced as a child persisted into adulthood, Michael said.
In a 2016 email to his father, Brendt told of nightmares in which he woke up yelling after seeing "something ominous." At other times he said he experienced sleep paralysis, where he was semiconscious, couldn't move and was "terrified and feel like someone is watching me," his father said.
Michael Christensen also described his ex-wife's alcoholism, which began when Brendt was in grade school and led to the end of their marriage years later. She sometimes would drink a quart of vodka or gin in a day, he said.
She once took Brendt and his older brother on an all-terrain vehicle and flipped it, Michael Christensen said. She also would drive drunk, often hitting curbs and hitting the garage and the house, he said.
"I could no longer trust her to take care of the kids safely," he said.
Michael Christensen, who splits his time between Michigan's Upper Peninsula and Minnesota, said he is staying at a campground while attending the trial, which is being held at a federal courthouse in Peoria. He is self-employed and makes $10,000 to $20,000 a year and can't afford a hotel for a month, he said.
Defense attorneys are presenting dozens of mitigating factors in an attempt to convince the jury that Christensen should be sentenced to life in prison rather than death.
During his cross-examination, Assistant U.S. Attorney James Nelson asked Michael Christensen about a phone conversation of July 4, 2017, days after his son's arrest in connection with Zhang's disappearance.