MC shouldn't have let her take revenge like this.
Unless someone is really strong, torturing someone will not do them any favor. And she was pretty broken to begin with, so this was the last straw.
As for the "victim" of the day, the flashbacks seem to point at some past genuine feelings that got distorted. Probably due to the influence of the mastermind behind the past bullying on the MC. He is sick and deserved retribution, but it's hard to tell if he was bad to begin with or what kind of person he would have been if he didn't get involved with that crowd.