Why does Peter add the word “zealous”?

Peter could have written simply, “Who is there to harm you if [when] you do good?” By adding the phrase “are zealous for” Peter shifts the focus from the doing of the good to the motivation. He sees his readers as leaning into doing good. Knowing that “harm” (of whatever intensity) may follow can prompt one to lean less into doing good. That is the danger Peter’s readers face. Hence the apostle’s effort through this letter to encourage them.