Question: Why did Jesus have to die?
It is quite likely that Jesus understood himself to be the Messiah of Jewish tradition, whose role it was to proclaim the coming of the Kingdom of God. Furthermore, it is likely that he expected to play an important role in that kingdom — indeed, that he would become king of the Jews.
It is the latter expectation that probably caught the attention of the Roman authorities. Contrary to much popular opinion, the Roman government was not intolerant of religion. There were many religions in the Roman Empire, and they were not persecuted — so long as they performed their duty to worship and make sacrifice to the official Roman gods. But the one thing the Roman authorities would not tolerate was the refusal to worship the official gods, or for someone to proclaim themselves to be king over against the Roman emperor.
Thus, from the perspective of the Roman authorities, Jesus—who was being proclaimed as King of the Jews—“had to die.” He had to be executed.
However, some of his followers reported experiences in which Jesus was alive again. As these reports spread among his followers, the stories those followers told one another began to evolve.
The letters of Paul which were written beginning around 50 C.E. reveal some of this evolving message. It’s in these letters that we first hear some of the themes that will become important in the later Christian message. The Gospels — written between 70 and 90-100 C.E. — continued the evolution of this Christian message. It’s important to note that the letters and the Gospels now used language to describe Jesus— such as “Son of God,” “Lord,” and “Savior” — that had formerly been used to describe the emperor.
With this evolution of the message about Jesus also came the proclamation that he “had to die” to bring about our salvation.
For further exploration, I recommend Marcus Borg’s Evolution of the Word. Borg has a clear and helpful explanation of how the message about Jesus evolved. He has also arranged the books of the New Testament in the order in which we believe they were written, thereby providing an interesting new perspective.