Walker said the pair had carried out a "senseless and unimaginable attack" which had taken away Pairama's will to live and presented her with an impossible choice.
But she said "sometimes we can never know why things happen".
Another teenager, who has been granted immunity from prosecution in exchange for giving evidence, will tell the court what she saw and heard at the Buckland Rd home, Walker said.
Police first began investigating after breaking up a fight near Britomart in central Auckland.
Walker said the group who were at the house had turned on each other.
"You'll pay for what you've done!" Winter was told, the court heard.
Walker said police would soon discover the "whole horrifying truth" about how Pairama, who had spent time in Oranga Tamariki care, was killed.
The prosecutor said Winter has since "told countless lies" to police.
The court heard Winter has, at various points, claimed she was Pairama's sister, admitted she played a part in the killing, said there was no ringleader, and that she had a reason to be angry at the teen.
Pairama was described by those her knew her as a "bubbly, pretty and very friendly" girl who had a distinctive laugh, the court heard.
Her life goal, Walker said, was getting a qualification and enough money to buy a house.
Justice Brewer earlier told the jury in a case such as this it may be "easy to feel prejudice against people who have been brought to court for charges like these".
But he said this was "not a court of morals, it is a court of law".
The judge urged the jury to decide on what the Crown has proved before considering their verdicts.
"In deciding what happened you are going to have to make human assessments … some people can be honest and some can be honestly mistaken," Justice Brewer said.
"You must keep an open mind until you have heard all of the evidence, the lawyers' closing arguments and my summing up.
The judge said: "Above all Mr Foreman and members of the jury, your job is to be fair."