Knox, of Seattle, wanted to get back at Kercher for saying she was not clean and for calling her promiscuous, Mignini said.
"Amanda had the chance to retaliate against a girl who was serious and quiet," Mignini said. "She had harbored hatred for Meredith, and that was the time when it could explode. The time had come to take revenge on that smirky girl."
He said Knox, Sollecito and Ivory Coast citizen Rudy Hermann Guede met at the apartment where Kercher was killed on Nov. 1, 2007, shortly before the slaying, likely to settle some drug issues with Guede, who was known in Perugia for dealing drugs. He said Kercher and Knox started arguing and then the three brutally attacked the Briton.
Kercher's body was found in a pool of blood the next day, her throat slit.
Knox and Sollecito are charged with murder and sexual violence in the 2007 killing in the central Italian town of Perugia. They deny wrongdoing.
Guede was sentenced to 30 years in prison last year on the same charges in a fast-track trial he was granted at his request. He also denies wrongdoing and is appealing his conviction.
Mignini recalled previous testimony by Kercher's friends, in which the Briton expressed surprise and irritation at Knox's behavior. Knox has denied having major problems with Kercher and has said in the past she was shocked at the death of a woman she considered a friend.
Mignini also said that Knox and Sollecito staged a burglary in the apartment by breaking a window in a bedroom in an attempt to sidetrack the investigation.
A rock was found in one of the bedrooms, and witnesses testified that shattered glass was found all over clothes on the floor, suggesting the window was broken after the room was put into disarray.
"The key to the mystery is in that room," Mignini said. It would be nearly impossible to climb through the window without getting cut and leaving blood on the shattered glass.
Also, he argued, that window was the most exposed of the apartment, making it an unlikely choice for a burglar. Nothing in the room with the broken glass, which belonged to one of Knox's and Kercher's roommates, was reported missing, Mignini noted.
"All of this was done to channel suspicions on a stranger, and divert them from those who had the apartment keys," he said.
Knox and Sollecito have been jailed for more than two years and appeared tense as they sat in court Friday.
Prosecutors were expected to formally make their sentencing requests to the eight-member jury Saturday, while a verdict is expected in early December. Knox and Sollecito could face Italy's stiffest punishment, life imprisonment, if convicted of murder.
Among the evidence brought by the prosecutors is a knife with a 6 1/2-inch (16.5-centimeter) blade found at Sollecito's house that they say could be the murder weapon.
According to prosecutors, the knife had Kercher's DNA on the blade and Knox's on the handle -- a claim defense lawyers reject, saying the knife is too big to match Kercher's wounds and the amount of what prosecutors say is Kercher's DNA is too low to be attributed with certainty.
The 22-year-old Knox maintains she spent the night of the murder at Sollecito's house in Perugia. The 25-year-old Sollecito has said he was home working at his computer that night. He said he does not remember if Knox spent the whole night with him or just part of it.
Defense lawyers for Knox and Sollecito are working on the theory that Guede was the sole attacker.