The very fact that we are sinning human beings under judgment disqualifies us from judging. Our manner of life in the past has so perverted our judgment that we are incapable of judging with the fairness of God. Our judgment is too subjective to be fair, too influenced by our own experiences to consider all the nuances of another's life to judge without prejudice. Not until after we have lived a life of overcoming and are rid of this body and mind of flesh will we be in a position to judge the lives of others.
Since we are obviously empowered to judge between right and wrong and commanded to choose the right even when evaluating the conduct of others, the judging that God forbids is the passing of judgment against another. In other words, God forbids the handing down of a sentence. It is one thing to call a spade a spade and decide that such an act is evil, but to condemn the person as evil, implying incorrigibility, is stepping into the minefield.