One of two men convicted in a series of random nighttime slayings that terrified this desert city should be spared the death penalty because he tried to right his wrongs, a defense lawyer argued Wednesday.

Jurors heard from attorney Maria Schaffer at the beginning of the penalty phase trial for Samuel Dieteman, who pleaded guilty to two murders in metro Phoenix's Serial Shooter attacks in 2005 and 2006.

Dieteman was the key witness against co-defendant Dale Hausner, who was sentenced to six death sentences earlier this year for the random 14-month shooting spree that left six people dead and 19 others injured.

Prosecutors said they would still pursue the death penalty against him despite his guilty plea and testimony. Dieteman is hoping jurors will spare him because of his cooperation. The penalty phase of his trial is expected to take more than a week.

The dozens of attacks and a second series of serial killings unnerved Phoenix as hundreds of investigators tried to crack the case. Prosecutors said Hausner preyed on pedestrians, bicyclists, dogs and horses during the 14-month conspiracy that occasionally included Dieteman.

Schaffer assured jurors that sentencing her 33-year-old client to life in prison would not be letting him off easy.

"His life will be hell either as a lifer or on death row," she said.

She showed jurors a picture of Dieteman well before the attacks, standing on a beach with a dog and repeatedly told them her client was a loved son, husband and father.

Schaffer said Dieteman spiraled out of control when he met Hausner, 36, saying Dieteman was "living under the awful specter of Dale's influence."

Prosecutor Vince Imbordino began his opening statement by meticulously describing various Serial Shooter attacks and Dieteman's behavior.

"Sam Dieteman was drinking and using drugs and he was shooting people or he was with Dale Hausner while he was shooting people," he said. "That was the majority of his life."

Imbordino also showed jurors graphic pictures of the victims' wounds and talked about the pain they would have experienced.

Authorities say Dieteman and Hausner preyed on pedestrians, bicyclists and animals in attacks that ended in August 2006 when both men were arrested at the apartment they shared in Mesa. Inside, police found guns, news clippings of the killings and a city map marked with the locations of some of the shootings.

As the serial killings were occurring, police attributed 23 more attacks, including nine slayings, to an assailant dubbed the Baseline Killer.

Mark Goudeau was arrested and charged with those killings in September 2006. He was convicted of two sexual assaults authorities linked to the Baseline Killer, but Goudeau still faces trial on nine murder counts. He has pleaded not guilty.