FORMER model Traci Vaillancourt was once a drop-dead-gorgeous girl with a promising future. But her life spiraled out of control after a rare blood disorder robbed her of her looks – and now she’s awaiting trial in a coldblooded cop-killing case.

After struggling for years with addiction, homelessness and the heartbreaking transformation of her appearance, the 34-year-old hit rock bottom over Labor Day weekend when her drug-crazed boyfriend allegedly killed a police officer before turning the gun on himself and Traci.

“It’s a tragedy,” said Traci’s sister, Vicki King. “We tried and tried to help her – but you can’t help someone who won’t help themselves.”

The heartbreaking story unfolded after Traci and her boyfriend, Timothy Walker, 35, were stranded on the side of the road in Draper, Utah, near Salt Lake City.

Walker, allegedly high on meth, had flattened the passenger-side front tire by driving their Volvo over a curb, a stop sign, a large rock and then through a fence.

Draper Police Sgt. Derek Johnson pulled up to see if he could help the stranded motorists. But before he even got the chance to exit his cruiser, cops claim Walker shot him through the open front passenger-side window.

As Johnson, a 32-year-old father of a young son, tried to drive away despite a gunshot wound to his chest, Walker allegedly continued firing at his car. The critically wounded officer radioed for help but lost control of his vehicle and crashed into a wooded area about two blocks away before dying.

Meanwhile, Walker allegedly shot Traci in the back and then turned the gun on him­self. Both survived, and Walker was charged with first-degree felony ag­gravated murder plus attempted homicide. Traci wound up with two felony counts of obstruct­ing justice for lying to investigators about the tragedy.

Vicki said her sister Traci had struggled with addic­tion to prescription drugs for nearly a decade – ever since she started taking painkillers to deal with swelling from a blood disorder that also destroyed her cover-girl looks.

“She would cry, ‘I never pictured my life turning out like this,’” recalled Vicki, who added that she knew some­thing was off when Traci and Walker showed up at her house the day before the killing. Vicki said the two appeared to be on drugs, and Walker showed off his arsenal of weapons – daggers in his socks, a machete in the side pocket of his backpack and a gun. She gave them $4.50 from her purse, and the two left. The following day, cops say Walker gunned down Sgt. Johnson in cold blood.

More than a thousand mourners showed up for the beloved officer’s public viewing. “There’s been a huge outpouring from ev­eryone in the community,” said Draper Police Chief Bryan Roberts.

District Attorney Sim Gill added: “Sgt. Johnson wanted to do nothing but help out. The death penalty is on the table in this case.”