In his decision to take the death penalty off the table, Judge Ronald Kessler said King County Prosecutor Dan Satterberg "relied upon a flawed, practically useless" investigation into the mitigating circumstances in the case.
Monfort is accused of gunning down Seattle police officer Timothy Brenton in his patrol on Halloween night, 2009.
Brenton was parked in Seattle's Leschi neighborhood after a traffic stop when Monfort drove alongside and opened fire, killing Brenton and grazing the neck of his partner, Officer Brit Sweeney, investigators said.
Prosecutors said Monford was engaged in a "one-man war" against Seattle police. He is also accused of attempted murder in the firebombing of several police vehicles in a maintenance yard.
Police also said he had been preparing to make a last stand at his Tukwila apartment complex when detectives shot, paralyzed and arrested him.
On Friday, Kessler said Satterberg's investigation of the mitigating circumstances in the case were "flawed" and that Satterberg abused his discretion "substantively and procedurally."
"The only rational remedy the court can impose is to strike the notice of intent to seek the death penalty," Kessler said.
In an effort to keep the case moving, Kessler said the trial will continue as a capital case.
"This court's decision today should not delay in any way the process of this case be it capital or not," Kessler said.
Earlier this month, Monford's lawyers filed their notice of intent to rely on the insanity defense.