The killer was arrested after buying a drink from a drive-through fast-food restaurant on the way to kill more people.
That was five years ago. On the stand inside a courtroom today, the miraculous survivor of one of Texas' most confronting crimes never flinched, reports news.com.au.
She gave evidence on day two of the trial for her uncle, Ronald Lee Haskell Jr, the man accused of lining up Cassidy, her four siblings and her parents and firing 13 shots, six of which were fatal.
Cassidy Stay reacts as a first responder tape is played during the trial. Photo / AP
Cassidy, aged 15 at the time of the shootings, lost her brothers Bryan, 13, and Zach, 4, her sisters Emily, 9, and Rebecca, 7, and her parents, Katie and Stephen Stay.
Now 20, the Utah-based university student was asked to confront the man accused of taking her family from her.
During a highly anticipated day in court, Cassidy said she remembered everything.
"The room smelled like blood. It tasted sour," she told the court of the day in 2014 when "uncle Ronnie", dressed as a FedEx deliveryman, busted through the front door and inflicted unthinkable horror.
"It felt heavy and hot. There was no spirit in that room," Cassidy said.
Aurielle Lyon, sister of Katie Stay, identifies Ronald Haskell during the second day of his capital murder trial. Photo / AP
Wearing a white dress, blue cardigan and glasses, Cassidy revisited the moment Haskell, 39, walked in with a 9mm pistol and a silencer made from a white pillow wrapped in duct tape.
"I was trying to appeal to his humanity," she said. "I didn't think someone would hurt kids if he knew their names and how old they were."
Cassidy said she told Haskell: "Please don't hurt us … because I knew how awful he was."
The 20-year-old said her mother realised what Haskell was going to do and jumped up and screamed "No" and began to fight him. Haskell then shot and killed Katie Stay.
"I looked down and I covered my ears and I started screaming."
She said the shooting occurred in rapid succession, "boom, boom, boom", as Haskell carried out the murders methodically.
Ronald Haskell, center, appears in Judge George Powell's courtroom. Photo / AP
The court yesterday heard the 911 recording Cassidy made to police.
Houston Chronicle reporter Samantha Ketterer, tweeting live from court, wrote the call included "ragged breathing and moaning from Rebecca Stay, 7, the only person besides Cassidy who was still alive at that point".
She wrote the TV was on in the background, "presumably still on from when Cassidy needed to distract her siblings from Haskell".
We got right into the 911 call. Cassidy Stay was frantic, and in between bouts of crying and coherence. She heard someone breathing, but not sure who.
"I don’t know what’s going on or why he did this."
The court heard Haskell told the family to lay in a row before the shooting began, and as Cassidy played dead, "she heard a drip" and "realised it was her sister's blood dropping onto the floor".
Haskell, who was looking for his estranged wife — Katie Stay's sister — is claiming insanity. His lawyers are trying to argue he heard voices that day and was not himself.
To that, Cassidy was unequivocal.
"No, it was all him," she said before wrapping up her evidence.
If found guilty, Haskell faces the death penalty.
Texas is one of the 29 US states where capital punishment still applies to those who commit murder.
