Looking back on Walker's behavior Saturday afternoon, King recalls details that are chilling in retrospect but at the time seemed characteristic of the dysfunction that estranged King from the couple in the first place.
Walker, 35, and Vaillancourt, 34, appeared to be on drugs when they arrived, King said. Vaillancourt had struggled with addiction for nearly a decade, since she started taking painkillers to deal with swelling from a blood disorder. Walker, meanwhile, was talking "a mile a minute," King said.
Inside King's home, Walker was "excited" to show off his weapons, King said. Little knives or daggers were stashed in his socks, and a machete was in the side pocket of his backpack, King said.
"He said, 'Check this out!' and got out a gun," King said. Her teenage son's friend gasped. King said Walker retorted, "You'd understand if you had a wife."
Walker and Vaillancourt said they had been camping for awhile and brought the weapons for protection, King recalled. The two had been living in and out of hotels for years, she said. King gathered $4.50 from her purse, tossed it in a plastic bag for her sister and shooed them away. They drove off in a silver car she had never seen before.
"For years I have kept them away from my children. They chose a different path," King said.
Growing up in Sandy, Vaillancourt had been "drop dead gorgeous" and had done some modeling, King said. But about a decade ago, she began to slip deeper into addiction, King said.
"She would cry, 'I never pictured my life turning out like this,'" King said. Walker gave Vaillancourt attention, King said, and Vaillancourt "was attached to him at the hip."
Five years ago they had a daughter, who has lived most of her life with her grandmother, according to court records. Shortly after the girl's birth, Vaillancourt suffered a series of strokes that left her impaired, King said. In 2009, Walker signed an agreement granting temporary guardianship of the girl to the grandmother; on July 31, 2013, the grandmother received permanent custody. King said Walker and Vaillancourt never responded to the custody notices.
In the meantime, the couple have been in and out of legal trouble. Walker was charged in February with three counts of possession or use of a controlled substance and had a warrant for missing an Aug. 7 court date. He also pleaded not guilty in December to charges of labeling or packing a controlled substance. Walker was convicted in 2008 of using plates registered to another vehicle, operating a vehicle without insurance, driving on a denied license and failure to appear on a citation. In 2003, he was convicted of driving on a suspended license and no proof of insurance.
Vaillancourt was charged Aug. 20 with misdemeanor shoplifting. She also pleaded guilty in 2004 to third-degree felony theft in exchange for the dismissal of six other misdemeanor counts of theft by deception.
"We tried and tried and tried to help her," King said. "But you can't help someone who won't help them self."
On Sunday morning, King saw a picture of a silver Volvo in news coverage of a lockdown in Draper.
"I had a bad feeling," she said. She told officers that it looked like the car Vaillancourt and Walker were driving. She gave a description of the couple. It matched, she said.
Police reported that Johnson, 32, pulled up to the silver Volvo, which was parked at an odd angle at 13200 South and Fort Street (900 East). Soon thereafter, police said, the man in the Volvo pulled out a gun and ambushed Johnson, shooting the officer as he sat inside his police car.
Johnson later was pronounced dead at Intermountain Medical Center.
At some point, Walker apparently shot Vaillancourt and then himself in an apparent attempted murder-suicide, police have said. Unified Police Lt. Justin Hoyal said Tuesday that the two remained in critical condition at Intermountain Medical Center, both suffering from gunshot wounds. Hoyal declined to further specify their injuries, but King said she was told that her sister was shot in the back.
"I know she was a victim, but I felt like I already lost her a long time ago. My heart is absolutely bleeding for Derek Johnson's family," King said, choking back tears. "I just want them to know I'm sorry."
The motive for the shooting remains a mystery "and we won't know until our detectives are able to interview [Walker and Vaillancourt]," Hoyal said. He could not estimate when the two might be able to answer questions; Salt Lake County Sheriff Jim Winder said their injuries are not life-threatening, and police are proceeding as if they both will survive.
Draper Police Chief Bryan Roberts said Vaillancourt and Walker appeared to have been living in the car, which King said was packed full of belongings on Saturday.
The Volvo is not reported stolen, though Winder declined to say at this point why two transients were in possession of a higher-end vehicle. King said she believed a friend had sold the car to them. Investigators began searching the car for evidence Tuesday morning.
The handgun is not reported stolen either, Winder said.
Johnson had been a full-time Draper police officer since 2004. He was promoted to sergeant in December 2012.
Johnson's funeral will be held at the Salt Palace Convention Center, 100 S. West Temple, on Friday, Sept. 6, at 11 a.m., and a viewing will be held Thursday, Sept. 5, from 6 to 9 p.m. at the Salt Palace, according to Draper City officials. Salt Palace officials confirmed that Comic Con, expected to draw about 40,000 revelers Thursday through Saturday, also is expected to go on as scheduled. The comic book convention and the funeral will take place at opposite ends of the convention center, officials said.
Interment will be at Larkin Sunset Gardens, 1950 E. Dimple Dell Road (10600 South) in Sandy, according to staff at Larkin Mortuary.
Johnson's family received a donation from the Utah Law Enforcement Memorial Foundation on Tuesday. The group primarily maintains the officer memorial at the Utah State Capitol, but also collects funds to help the families of fallen officers.
Draper City also announced that there are two ways for the public to donate. Any Wells Fargo Bank branch accepts donations to the Sergeant Johnson Memorial Fund, and people can donate online at mydadsahero.com.
Reporter Michael McFall contributed to this story.