Why did Peter say the Jewish rulers killed Jesus in ignorance?
Peter said it to leave Israel an opportunity to recognize that the One they killed was the Promised Messiah, emphasizing their act was done without understanding. This aligned with Christ's prayer, "Father forgive them, for they know not what they do," and Stephen's similar plea.
Les explains that Peter was still coming back to Israel, hoping against hope that Israel would wake up and realize that the One they killed was the Promised Messiah. Peter addresses them as brethren, showing he was speaking to Jews, not Gentiles, at that point. He says they killed the Messiah through ignorance, as did their rulers. Les notes this raises the question of why Peter gave Israel this option, and connects it to Christ's words from the cross, "Father, forgive them, for they know not what they do," and Stephen's similar statement in chapter 7.