List's defense attorney, Elijah Miller, pressed for a change of venue and suppression of the confession letter, but he had to do this in the name of John List, not Robert Peter Clark, so the insistence that the state prove that the two were one and the same was dropped. In his opening statement, he conceded that List had done what he was accused of doing on November 9th, 1971. However, he had a fragmented, obsessive-compulsive personality and was ill equipped to function in the face of overwhelming difficulties. He had acted, he believed, for the salvation of his family in a Godless world that no longer made sense to him.
Eleanor Clark, the prosecutor, described him as cruel and calculating to the point of evil. He wanted a new life so he simply closed the accounts on the old one and left it behind. He had voluntarily committed the murders with full knowledge and with malice. The fingerprints, ballistics reports, autopsies and his own letters indicted him beyond a shadow of a doubt.
The defense entered information that Helen List had been suffering from syphilis, acquired from her first husband. John had found out about it in 1969, after she had successfully hidden it from him throughout their marriage. This piece of information failed to have any mitigating effect.
The prosecutor brought in a psychiatrist who rebutted the defense's idea that List had a character disorder, calling it a situation depression from a midlife crisis. He talked about how List believed that if he killed his family, he was sending them to heaven, but he could not kill himself because that meant going to hell. He claimed that once he started killing, there was no more control. Once he left, he put it behind him and finally reached the point where he only thought about what he had done on the anniversary of their deaths.
The trial on five counts of murder in the first degree lasted seven days. On April 12, 1990, List was found guilty on all counts. He was given life in prison, five consecutive terms. He would never be released.