The girlfriend of the accused Bourke Street killer said he vowed to run down people in the Melbourne CBD before she escaped from his car.

Akiir Muo said she was kidnapped by Dimitrious "Jimmy" Gargasoulas, 26, from his Windsor home on Friday morning because she had witnessed his brother's stabbing.

Ms Muo said she feared for her life as she was driven around by Mr Gargasoulas.

The 25-year-old mother of two said her on-off boyfriend of about five months had stabbed his brother and then threatened to kill her, his mother and himself.