Today's parable is about violence. Jesus tells a story of evil actions (multiple murders) that would arouse feelings of anger, retribution and violence in just about anyone. But, of course, once he finishes the story and has aroused these feelings in his listeners, he uses them to instruct and teach.
One way to interpret this story is that, metaphorically, God sends us messengers, emissaries, and prophets, and we methodically kill them. These emissaries come in many forms and shapes, and we seem to always react the same.
But there is no violent retribution from God. God is not "in our image" full of violence and revenge. We are "in God's image" and are urged to build on a new cornerstone and to pursue justice and mercy through Jesus.