...you killed the Author of life, whom God raised from the dead. To this we are witnesses.
Acts 3:15

Peter should have been a witness at Jesus' trial, but having repented of that sin, he did what he was able: He witnessed to Jesus' resurrection and the eternal life it enabled.

The King of Life came to give life to Peter and all the Jews, but the Jewish leadership rejected him and gave him death. That's a good thing for everyone else. Without his death and resurrection, Jesus could not have given life. They didn't understand what they were doing, but He did, and He forgave them.