"I did not expect this to happen," Knox told
The 26-year-old shared a house in the Italian town of Perugia with Kercher, then 21, who was found partially naked in a pool of blood, her throat slashed.
Knox and her former boyfriend, Raffaele Sollecito, 29, were convicted of the crime in 2009 and had spent four years in prison before their acquittal in 2011.
However, Italy's highest court overturned the acquittal and ordered a new appeal, saying the first was riddled with "shortcomings, contradictions and inconsistencies."
Though she was not in the Florence courtroom Thursday when Judge Alessandro Nencini sentenced her to 28 years and six months in prison, more than the 26 years she received at her first trial in 2011, she told Roberts she watched an Italian television station online to hear the verdict.
"I needed to hear it for myself," she said. "My whole family was there and I was listening and I'm the only one who knows Italian and I'm trying to listen and then tell them."
Knox had refused to attend the second appeal, which began last year, writing to the court from her hometown of Seattle that she feared being "wrongly convicted."
She choked up when Roberts asked about her ex-boyfriend, Sollecito, who was also found guilty and sentenced to 25 years. He was instructed to hand over his passport and forbidden from leaving the country before Italy's supreme court confirms the sentence.
"I don't know what I would do if they imprisoned him," she said. "It's maddening."
Both in the interview and in a statement released on Thursday, Knox expressed her condolences for the Kercher family.
She said she had sent a letter to her lawyer that is addressed to the family of her former roommate.
"Mainly I just want them to know that I really understand that this is incredibly difficult, that they've also been on this never-ending thing and when the case has been messed up so much, like, a verdict is no longer consolation for them," she said.
This echoed her statement Thursday, which read: "Their [the Kercher family] grief over Meredith's terrible murder will follow them forever. They deserve respect and support."
In a shaky voice Knox said she is "going through waves of emotion. ... My first reaction was, no, this is wrong, and I'm going to do everything I can to prove that it is."
To help cope with her feelings, Knox said she reached out to the priest who counseled her when she was in prison.
"He's reminded me that people still believe in me and that this is an experience I have to testify to, that really horrible things can happen and you have to stand up for yourself," she said.
If the guilty verdicts are upheld, Knox could face extradition proceedings. She told GMA she is not prepared for that.
"I will never go willingly back to the place where I ...," she said, pausing. "I'm going to fight this to the very end. It's not right and it's not fair. ... I'm going to do everything I can."