Video showing Minaj, 29, screaming and cursing at Carey, 42, on the set of "American Idol" made the rounds last week. The recording artists joined the singing competition series as judges this season.
"How dare this little girl! ... If I had been Mariah I would have walked over to Nicki and strangled her to death right there," Nicks, 64, told The Daily in an interview to promote her new documentary, "In Your Dreams."
"I would have killed her in front of all those people and had to go to jail for it," added the former Fleetwood Mac lead singer.
But Nicks later backed off the statement.
"I want to apologize for my remarks about Nicki Minaj's behavior toward Mariah Carey, which I said during a long and exhausting day of interviews," Nicks told the New York Daily News. "It was very out of character for me and I deeply regret what I said. I feel very protective toward Mariah Carey who has gone through many difficulties in her life and I spoke without thinking. I think all artists should be respectful toward one another and that includes me."