He stopped the truck, got out, retrieved a rock and struck Eitner in the head with it.
Ashton then dropped the other man off at his own home and picked up another individual who is identified in court documents as his "accomplice."
The two drove to a secluded location in the Arizona desert and helped Eitner, who was still alive at that point, out of the truck.
"My accomplice then shot Mr. Eitner twice with a rifle, and at least one shot hit Mr. Eitner in the head," Ashton said in a plea statement. The accomplice pushed Eitner off an embankment, where he became wedged in a rock crevice.
Eitner's body was discovered on March 11, 2011. An autopsy determined he died as a result of blunt force head trauma and a single gunshot wound.
Ashton entered guilty pleas to kidnapping resulting in a death and aiding in the use of a firearm during a violent crime.
Federal prosecutors will recommend that Ashton get life in prison for the kidnapping charge and 10 years for the firearm charge, both the mandatory-minimum sentences allowed by law, and that the penalties run consecutively. A sentencing date has not yet been set.
Ashton is facing two counts of aggravated murder, a first-degree felony, and other charges in connection with the Dec. 11, 2010, murders of Brandie Sue Dawn Jerden, 27, and Jerrica Christensen, 20. That case is pending.
According to state court documents, Christensen and 28-year-old James Fiske were helping Jerden and her boyfriend move out of Ashton's townhouse after he accused them of stealing items. Tension escalated and prosecutors allege Ashton killed Jerden and wounded Fiske, while Brandon Perry Smith killed Christensen to silence her.
Aric Cramer, Ashton's attorney, had said previously that Ashton, who was using a wheelchair at the time because of a fractured leg, reacted after Jerden hit him in the head with a toolbox.