Why would killing someone make up for sins?

The difference here is that when people were sacrificed, it was being done consciously....those people knew what they were doing and for what reason. When Jesus was crucified, the Jews did not "really" know what they were doing - it wasn't being done with the thought that our sins will all be forgiven - it was done (according to the story) because of the Jewish high priests didn't want a rabble rouser.