Christian: People were giving sacrifices in the Old Testament for forgiving their sins, so Jesus was the sacrifice so that people don’t need to do that anymore.
Muslim: But who killed Jesus? Were they sinned people who wanted atonement? No, They were the Jews who fought Jesus and killed him as an opposition to his message. So they were not actually giving atonement, they were sinning. How can a sin be an atonement for a sin? An atonement is supposed to be a good deed from a man who repented for his sin not from a man who is sinning by this atonement.