In a wide-ranging interview with PEOPLE, Grigorieva, 40, talks in detail about the argument during which she says Gibson, 54, hit her while she was holding their baby, Lucia.
"I thought he would kill me," says Grigorieva. (Gibson, who declined to comment for this story, has disputed her account and a source close to him has said he intervened when she was shaking the baby.)
The Russian-born musician says Gibson often flew into jealous rages, talked about suicide and once brandished a gun in front of her.
She also discusses why she decided to tape Gibson's angry phone calls. "I needed to document his threats," says Grigorieva, who denies she leaked the tapes. She also refutes Gibson's claim that she used the tapes to extort him.
"I'm not angry at him," she says. "For the sake of Lucia, I really want him to be well. He has to have the courage to be responsible for what he's done."
For much more about Oksana Grigorieva's relationship with Mel Gibson, look for the next issue of PEOPLE, on newsstands Friday