Only the person against whom the crimes were committed can forgive
In order to forgive someone, they must prove themselves worthy of forgiveness
I am so sorry for what I did. I fixed the vase and bought you some new flowers
I forgive you because you have worked very hard to earn my forgiveness
Forgiveness can only be awarded by the person who was hurt.
I feel pretty bad about what I said to her, and I'm sorry. Can you give me forgiveness?
Not exactly. Only the person that you've hurt can truly give you forgiveness
Religion impacts how people respond to morally conflicting situations.
You should not forgive
You should forgive and forget
One theme of The Sunflower is that a person cannot be forgiven until they are truly remorseful of their actions. Karl makes a confession on his deathbed, and went out of his way to seek out a Jew to apologize to. This shows regret for his actions, and could be a reason for Simon to forgive him. If someone does not display this desperate remorse, they do not deserve forgiveness.
In order to be fully forgiven, a person has to get forgiveness from the person they hurt. A stand-in for this person does not work, as it has to be the faulted party. Karl dies without closure because the people he needed to get forgiveness from are dead. Simon could never fully forgive him, as he is simply a replacement.
In The Sunflower, Simon grapples over whether he should have forgiven Karl or not. He asks many people, and their responses vary. People that are Christian told him to forgive. Karl repented, and is worthy of forgiveness. Other Jews that he talked to believed that Karl's crimes were so heinous, he was right in not forgiving him.