His 19-year-old cousin, Roy Coffey, was also charged with obstructing justice in the slaying for allegedly fleeing the scene and tossing the murder weapon into the Great Salt Lake.
The violent night began, investigators have said, when Horowitz and a dozen other boys gathered at the elementary school to watch a fistfight between two boys who were competing for the affections of a girl.
Coffey, charges state, was one of those two boys.
But that fight was settled when Beach allegedly turned and began yelling obscenities at Horowitz and his friends.
Horowitz yelled back. Before long, charges state, the two young men were throwing punches and wrestling each other to the ground.
Witnesses told police that Horowitz was "getting the better" of Beach before Coffey threw himself into the fracas in an attempt to break the men apart.
Once Beach was back on his feet, court documents allege, he ran back at Horowitz, tackled him and then grabbed him around the neck.
"Some of the witnesses said that they could see [Beach] making stabbing motions with one hand while he had the other hand around the victim's neck," according to charges. "Witnesses stated that the victim went limp during the struggle, and as they tried to break up the fight, they discovered that the victim had been stabbed multiple times."
Beach is originally from San Antonio, Texas. He had been staying with his cousin, Coffey, for about a week, witnesses told police. None of the boys at the fight Friday night knew him, though, they said, he had been bragging about being a member of the Bloods.
Once Horowitz fell lifelessly to the ground, Coffey and Beach fled.
Horowitz's friends gathered around him and tried to help. One boy called 911.
Horowitz was dead by the time police arrived.
Both suspects later admitted to investigators they knew "something was very wrong with the victim," according to court documents.
As Coffey drove back toward his home in Tooele, court documents state, he asked his cousin "why did you do that?"
Beach allegedly responded: "Because he hit me in the face and I fought back."
Beach's motivations have been called into question by the NAACP of Salt Lake City, who wants to learn more about what happened that night.
Horowitz was black; the rest of the group was mostly, if not entirely, white, said Jeanetta Williams, president of the local NAACP branch.
According to court documents, Beach admitted to police several times that he "did it."
Coffey told investigators that Beach denied stabbing Horowitz as the two rode home from the incident.
Both were arrested Saturday when police executed a search warrant at their Stansbury Park home, where they also found bloody clothing and the white Toyota in which the boys had allegedly fled.
If convicted, Beach could face up to life in prison for first-degree felony murder and up to 15 years in prison for second-degree felony obstructing justice.
Coffey faces a maximum of 15 years behind bars.
The two men will make their initial appearances before 3rd District Judge Robert Adkins on May 5.