Mia’s conscience wouldn’t allow her to kill someone. She hates the idea of killing and fighting and gunplay. Played by Jordana Brewster in the movies, she’s the sister of Dom Toretto and mother of Brian O’Conner’s son and she barely lets them commit crimes, so forget about her ever committing
She turns a blind eye to her family’s work as mercenaries, but she would never get involved herself. She wanted nothing to do with the team’s revenge plot against Deckard Shaw. She did help out with the heist in Rio, but there was no killing involved in that.