Why did Stephen call the Jewish religious leaders betrayers and murderers of the Messiah?
Stephen charged Israel's religious leaders with repeating their fathers' pattern: resisting the Holy Spirit, persecuting and killing the prophets who foretold the Just One, and then betraying and murdering that promised Messiah. Their guilt was corporate and historical, not a minor offense.
Les explains that Stephen is showing them the same pattern their forefathers followed. The prophets came to their forefathers, and they killed them; Jeremiah, for example, was put in a deep, wet dungeon because he prophesied things Israel did not want to hear. Stephen then applies this directly to the religious leaders: they slew those who showed beforehand the coming of the Just One, and they themselves became the betrayers and murderers. Les notes that this was not kind language, but it was exactly what Israel was guilty of—they had killed their Messiah.
Which of the prophets have not your fathers persecuted? and they have slain them which shewed before of the coming of the Just One; of whom ye have been now the betrayers and murderers: