Thanks DHK, I think this verse that you cited is a key one:
This cry by Stephen is very similar to Jesus request to the Father while being nailed to the cross: "Father, forgive them for they know not what they do". Neither Jesus nor Stephen could grant forgiveness to a person who was in the act of sinning and was totally unrepentant. What they could do is release their persecuters to God's soveriegn control with a prayer to God that he would forgive them. I think that the family who loses a loved one to a murderer, in the same way, cannot grant forgiveness to the murderer, but can release the murderer to God's sovereign control with a prayer that God will forgive that person by working in their life to bring them to repentance. We also have confidence that God will meet out justice to those who do not repent. And God will wipe away every tear from the believer's eye. In the case of Jesus, we know that God did work in the lives of some who crucified Jesus and he brought them to repentance as recorded in the book of Acts.On the other hand when a Christian's forgiveness for others is directed to God it may be unconditional, such as Stephen's was. Stephen asked God: Acts 7:60 And he kneeled down, and cried with a loud voice, Lord, lay not this sin to their charge.