What Is the Lesson of the Death of Egypt's First-Born Sons?

The most severe of the Ten Plagues and therefore it was the last. But it was the most important (and most severe) because of the very great principle which it came to demonstrate, a principle which is second to the principle that Hashem created the Universe: the thesis that Israel is most important in Hashem’s eyes. "My son, My first-born is Israel” (4:22). —  A Nation is Born