In court on Thursday, Thompson pleaded not guilty to four charges in the fatal stabbing of William "K.C." Wright.
Thompson, 29, is charged with first-degree felony murder, second-degree felony obstruction of justice and third-degree felony counts of possession of a firearm by a restricted person and possession or use of a controlled substance.
Judge Thomas Low ordered Thompson to stand trial on the charges after hearing testimony from several witnesses who were at the party in July.
It all started when a neighbor came to the house to ask the partygoers to quiet down, testified Carston Terry, a friend of Wright's.
The 15-year-old had come with an older woman. After Wright assured her they would be quiet and moved the party inside the house, the teen continued to sneak in and take beers from the home, Terry said.
He was asked to stop, told to leave.
"He kept trying to drink," testified Amanda Calton, another partygoer. "He was going and stealing beer, and he was being very disrespectful and rude. ... He got kicked out of the party a few times, because he wasn't welcome there. He wasn't old enough."
Amy Anderson, who was also at the party, said she and several others exchanged heated words with the teen until Wright intervened.
"He was trying to be the peacekeeper," Anderson said of Wright. "He was trying to explain to this little boy why he couldn't be there."
But it wasn't enough.
As the party began to wind down, witnesses said, the teen began to yell threats from across the street.
"The kid said his cousin was coming to kill all of us," Terry said. "We didn't really take him seriously."
Minutes later, a car came to a screeching halt in front of the house. Terry said he and Wright thought there had been an accident, that someone who had been drinking tried to drive home.
They went outside to investigate.
Thompson was seated in the front seat of the car as Wright began to approach, Terry said.
"K.C. had his arms out like, 'Hey, man, everything's fine. Everything's OK. Nothing happened,' " Terry said. "That's when I seen him Keenan get out of the car."
According to a probable cause statement, Thompson told police that a man rushed toward him as he exited his car, so he grabbed a knife and began "swinging at the male with his fists," the document states.
Terry testified Thompson did a "360-type motion" and spun toward Wright. He didn't see any blows exchanged, he said, but he heard the "ting" of metal before Thompson moved away from Wright and climbed back in his car.
John Clements, who called Wright his best friend, said he didn't see the fight either. He didn't know anything was wrong until he heard several women scream.
His best friend was lying in the street, Clements testified. Clements lay down next to him, lowering his cheek to Wright's face.
"There was so much blood," Clements testified. "I stayed there with him 'till he took his last breath."
Thompson told police that he didn't know whether he had cut Wright in the scuffle, but witnesses said he didn't stick around to find out.
Thompson and his younger cousin got into his car and drove to Spanish Fork, prosecutors said. Along the way, Thompson threw the knife out the car window and stripped off a layer of clothes he was wearing another shirt and pants and put the top layer in a Dumpster, according to court documents.
In Spanish Fork, the teen said, Thompson "freaked out" and said, "I hope I didn't kill him."
Wright died of stab wounds to his throat and abdomen, documents state.
Defense attorneys did not call any witnesses nor make any arguments against the charges, but they did raise the issue that Thompson is a Navy veteran who suffered "an incident on board an aircraft carrier that was very traumatic."
Defense attorney Craig Bainum said they are investigating whether Thompson's post-traumatic stress disorder may have played a role in the stabbing.
Thompson is scheduled to appear in court next on July 23 for a status conference.