One time I was at a Muslim lecture series and there was a young woman there who was with a Muslim a boy and was looking into Islam as a potential convert. One question she had, coming from a Christian background, was about the role of forgiveness in Islam. She noted that the topic of forgiveness did not seem to exist in the prevalent Islamic literature, whereas in Christianity it is a prevalent theme - we are forgiven for our sins, we obtain forgiveness by the grace of God, we are advised to forgive others. We have dhikr, "Astaghfirullah", seeking forgiveness, but it is hard to find much said about it, in how it is obtained or in the value of giving it.
But on the other hand, we do have lanaat. For example, from Ziyarat Al-Jami'a (imho, one of the most beautiful brief ziyaras), "May Allah (swt) curse the enemies of the family of Muhammad from amongst the jinns and humans and I dissociate by Allah from them...." And there are examples by name in Ziyaarat e Ashura.
The horrible events of Karbala beg the question of the role of forgiveness in Islam. Here were events in which people essentially set themselves up as enemies of God and Godly people and committed great oppressions against them culminating in massacre, imprisonment, and torture. The victims did not preach the perhaps misunderstood Christian concept of turning the other cheek. They taught and modeled standing up and fighting back against wrong. Imam Husain (as) did not go to Karbala to be a martyr, but he went because it was necessary to preserve Islam. He did not fight on personal principles, but on Godly principles.
There is the ideal in Islam of forgiveness, but the initial readings of someone new to the faith will not find it so easily as in Christian works. We are enjoined to overlook faults and to forgive shortcomings of one another. We are taught that when you wrong someone, God will not forgive you if you do not seek to fix that wrong so that the one you wronged forgives you, unless it is impossible for you to do so in which case you seek God's pardon on the victim's behalf. We are taught that God is Merciful beyond our comprehension, and that He makes our scales heavy with good deeds by accounting them more than their worth through His Mercy.
If we extend this idea that you must seek forgiveness from the one you wronged, we often need to seek forgiveness of ourselves but a great many of the sins we do our against ourselves. But they are also against God and so we have to seek His forgiveness. God said He may forgive anything except shirk, but we should be repentant and not use a belief in forgiveness as an excuse to persist in bad deeds.
When there is hope for reform, there is room for forgiveness. When it eases your soul, there is room for forgiveness. In most ordinary cases, forgiveness is a virtue that is good for you and for the one who harmed you. People are too stingy with forgiveness, but yet they expect Allah swt to not be stingy with forgiveness for them. Why not forgive?
When someone sets themselves up as an enemy of God, it is not our place to forgive. It is God's choice to judge. Our place is to enjoin good and forbid evil, to join good and to separate from evil. To bless good and curse evil. We must observe manners and decency and uphold rights of all mankind, even enemies. We must never sink to the denominator of our foes, because then they achieved a victory of corruption - of bringing us down to their level. No matter what is done, two wrongs never make a right. There is never an excuse to deny a human being his dignity, no matter what he may have done. The message of Karbala is powerful because the camp of Imam Hussain (as) unflinchingly maintained the moral right. They did not sink to the level of the oppressors. Had they engaged in even one small understandable act of anything but the highest moral virtue, then Karbala would have been a total loss for Islam and mankind. This is a message to take with us today and apply in our daily conflicts and to apply in the world conflicts.