Why did Jesus have to be killed by both Jews and Romans?
God's sovereign plan required it to fulfill Psalm 2. There, the heathen Gentiles and the people of Israel, along with Rome's rulers and Israel's religious rulers, take counsel together to reject the king. Christ's death therefore had to involve both Jew and Gentile, not just the Jewish nation.
Les explains that the sovereign God was in control, and Psalms 2 had dictated only one way for Christ to be put to death. It could not be at the hands of only the Jew; had the high priest let the people kill Him, that would have been only Jewish. It had to also involve the Gentile to fulfill Psalms 2 explicitly. In Psalm 2:1, the heathen are Gentiles, the non-Jew, and the people are Jews, so the whole human race is involved. Verse 2 shows the kings of the earth, which Les identifies as Rome's Caesars and rulers, and the rulers, the religious rulers of Israel. They take counsel together, working hand in glove to reject the king. It was not just a Jewish phenomenon or just a Roman thing; prophecy required both to participate.