Jason Dalton admitted carrying out the 2016 shooting spree in Kalamazoo County, Michigan, in between picking up fares for the taxi firm. Dalton, who was driving for Uber at the time shot eight people killing six of them, on Saturday, Feb. 20, 2016.
"After Dalton's arrest, police quoted him as saying a "devil figure" on Uber's app was controlling him on the day of the shootings," the AP said.
Solis said plea discussions had been ongoing for months, but that prosecutors dropped no charges and showed no support for a lighter sentence.
Four people were killed at a restaurant and two at a auto dealership in the shootings in the city of Kalamazoo.
Dalton abruptly pleaded guilty to murder and attempted murder - over his attorney's objections - triggering the prospect of a mandatory sentence of life in prison with no chance for parole.
Jason Dalton speaks with his attorney at the Kalamazoo County Courthouse on Monday, Jan. 7, 2019.
A 14-year-old girl was shot in the head during the restaurant shooting and survived, while an eighth person was injured in a residential area.
Fox 17 reported Dalton's attorney, Eusebio Solis, decided not to pursue an insanity defense late last week.
Dalton, the father of two children, had worked as an insurance adjuster and had no previous criminal record before the February 2016 shootings.
Dalton was not expected to plead guilty at the Monday morning hearing, with his lawyer revealing he had advised him against admitting to the mass killing.
But he told the judge: "There are personal reasons for him". Shop owner Jon Southwick recalled Dalton "laughing and joking", and giving a "one-armed hug" to the manager before making the purchase. But in the end, Dalton determined he "wanted to spare families more grief during a trial".