I feel like that scene could've worked, but they were framing it all wrong.
Instead of framing the scene as, "Oh no, Barbara's turning evil!" -- because, like you said. The guy was a monster and who cares if one of his potential victims kicks him to death, he really had it coming -- I think it would've worked much better if the scene was more about Barbara enjoying her new-found power. The focus shouldn't have been that Barbara had begun her descent into villainy, but rather, she's overjoyed to have this power and not be a helpless victim anymore, but rather, is now capable of taking all the pain she's suffered (by being ignored by co-workers?) and inflicting it back on the people who hurt her.
I don't know, I feel like they were trying not to portray her as too evil, you know. Keep her as a sympathetic character who's capable of being redeemed, but that weakened her whole journey as a person, and it just came across as petty and trite.