In reviewing my sources, it seems “atonement” originally is an Old Testament concept in which there is an action to appease god, usually an angry god. The principle method of appeasement was sacrifice. Primitive religions often used people (women and children preferred), but then animals, crops, other things could be substituted. However, a blood sacrifice of a person seems to work best in creating religious awe and hence community cohesion. My thoughts come directly from my limited understanding of the work of Rene Gerard and others with the Colloquium on Violence and Religion.
Israel tried to avoid the human sacrifice, and for me the story of Abraham substituting a lamb instead of killing a son is the sign for Israel that it was no longer okay to kill people to atone to their god. Israel, however, did provide cohesion not only with the sacrifice of animals, but with holy war and conquest.
The Old Testament indicates there were a few relapses to human sacrifice, but in the writings of some of the prophets there is a clear shift from the value of sacrifice as a means to worship god. God wants our hearts not our sacrifices was their message.
I do not think we know much of what Jesus actually thought or said, but I think he was more closely connected to the views of those prophets. He was against the rituals and the hypocrisy of the temple worship; for that he was murdered. To make sense of that murder, some followers of Jesus retuned to the mind set of their culture. Jesus’ death becomes yet another sacrifice, albeit the final sacrifice in the classical theological tradition. Rather, I see it as a revelation that an innocent man was murdered by the establishment and that God is on the side of the victim. Socrates’ death was similar in that it is recorded for what it was, but Jesus’ death has been transforming for Western culture because the truth was not covered up. The problem for me is that it is interpreted as yet another atonement to an angry god. I cannot worship that god, and my guess is that Jesus did not worship that god either.