That there will be a loud crying is understandable. Every Egyptian home sustained the loss of its firstborn. It is a normal response to sudden death. Why was it necessary to inform the Egyptians beforehand that there would be a great outcry? What else? Horav Yechezkel Levenstein, zl, explains that the Torah is teaching us an important lesson. The great outcry which would accompany the deaths of the Egyptian firstborn, an outcry which would seize and overwhelm the entire country, was not the result of a natural reaction to sudden tragedy. This outcry was part of the middah k’neged middah, measure for measure, punishment which the Egyptians deserved for how they treated the Jews.
When the Egyptians snatched the Jewish babies from the arms of their mothers, this cruel action certainly catalyzed a bitter outcry throughout the Jewish community. Therefore, the Egyptians deserved a punishment which would be appropriate in light of the distress they caused the Jews. The deaths of their firstborn was insufficient to compensate for the pain and grief experienced by the Jews when their children were thrown into the water. Hashem promised to judge the nation that afflicted His people. They would be paid back in full; nothing would be held back. Thus, they deserved to cry out with the same grief stricken screams that they caused the Jews to emit. Hashem is very exacting in His punishment: no more; no less.