If we have accepted Jesus as our Lord, we do not judge others. Jesus is the Judge of every person, and we would infringe on His lordship if we judged others. Of course, we can and must judge the morality of actions, but we should not hold court and sentence sinners. For example, we should report crimes to the police but not go so far as to take the law into our own hands.
Instead of judging sinners, we should love them enough to invite them to repent. We should pray and fast for them, do other penances for them, and serve them. If we love sinners, we hate the sins which are destroying them. Therefore, we must seek the Holy Spirit so that we can wage war on sin by bringing down the strongholds, lies, and pretensions of the evil one (see 2 Cor 10:4-5), especially through prayer and fasting (Mt 17:21; Gospel of Life, Pope John Paul II, 100).
We can gossip about sinners, complain about them, or reject them. Thus, we become worse sinners ourselves and continue to expand our culture of sin and death. Conversely, we can fight sin, love sinners, and build a new civilization of holiness, life, and love. Don't judge, and do fight.