Parshas Yisro: Ramban - Yisro is Impressed By the Revenge for The Drowning
Yisro heard all the stories of Yetzias Mitzrayim, but only after hearing them from Moshe does the Torah attest that he realizes the greatness of Hashem. Why? "Ki BaDavar Ashe Zadu Aleihem." Unkeles says that this means that Hashem paid back Mida K'Neged Mida because he drowned the Mitzrim just like they drowned the Jewish children. The Mitzrim committed many evils starting with enslavement, why was the drowning the sin that they were held accountable for and punished with?
The Ramban says that the word Zadu means, "sinister intentions". He explains that Hashem decreed that Bnei Yisrael would be enslaved. With regard to the slavery the Mitzrim were not going against Hashem's will. The reason why they were punished was because their intentions were evil. This was evidenced by the fact that they did far more than what was required. In their evil, they wanted not to enslave the Jews, but rather to annihilate them. To this end they plotted to throw the baby boys in the river.
Since their punishment was for the evil that was not part of the Gezeira, their end came through drowning which was the main crime they were guilty of. "This," says the Rambam, "is what Unkeles means that they were punished Mida K'Neged Mida. That Hashem understood the wickedness deep in their hearts and punished them accordingly, was what impressed Yisro."