Cameron "Wong" Wilson, 20, said Moore was violently stabbed with a knife as she tried to escape.
Asked why he did nothing to help her, he shrugged and said: “I don’t know.”
Instead, his “mind told him to run home and get into bed next to his girlfriend, where he fell asleep”.
Wilson, who is facing 15 charges including six counts of murder, is accused of raping and killing Moore in Valhalla Park in September last year.
He claims it wasn’t he but his friend Xavier Bester who murdered the girl.
Bester was shot dead hours after Moore’s mutilated body was found on a field in Tuna Road next to the 8ste Laan informal settlement.
Dressed in a check shirt, blue jeans and Air Jordan takkies, Wilson appeared cocky in the witness box on Monday.
Asked to describe in his own words Moore’s last moments, he said: “We were walking in 8ste Laan, and Moore was threatening Bester that she would lay a charge against him the next day."
“Next moment he snapped and stabbed her. She wanted to shout but no sound came out."
“Hy stiek haar toe aanhoudend. She fell and crawled away but he got her, covered her mouth and just started stabbing." (He stabbed her continuously. She fell and crawled away but he got her, covered her mouth and just started stabbing.")
“He then started undressing her. Hy het haar een tet gesqueeze and then he started slicing at her breasts and then her private part." (He then started undressing her. He squeezed her one breast and then he started slicing at her breasts and then her private part.)
“I just stood there in shock, I froze.”
But prosecutors argued the girl had multiple stab wounds to the back of her head, shoulders and back, indicating that she had been attacked while trying to run away.
“Mr Wilson, why are you telling the court this now? Are you afraid you will be found guilty and Valhalla Park residents will be angry?”
Wilson replied: “Dit pla my nou, it feels as if I’m hiding the truth. It’s time her family knew who killed her.” (It bothers me now, it feels as if I’m hiding the truth. It’s time her family knew who killed her.”
To which the prosecutor said: “I state that if you were telling the truth you would not have waited for people to testify or DNA experts to take the stand.”
At this, Wilson clenched his teeth and scowled at the prosecutor.
The trial resumes on Tuesday.