As Jennifer Garner explained in our interview, she enjoyed the fantasy element of Peppermint (which focuses on a woman who takes up arms against a drug cartel that murders her husband and daughter), but she never personally agreed with the methods or results that her character would get.
Similar to classic Greek tragedies, it just makes for good drama to tell these types of tales.
So, you don’t necessarily have to agree with revenge as a concept to indulge in the fantasy.