1- Those who were believers but had sins and shortcomings, we cannot disrespect them even after they die. We ask Allah for forgiveness.
2- Those who had no faith, did not repent and were know to be enemies of Allah and the Ahlulbayt (a)—like Yazeed—the Qur’an commands us to curse them. If we don’t curse them it means we are ok with their injustice. We cannot fully stand with justice without condemning them.