A. Because they sought to destroy Israel, who are called 'My firstborn son' (Exodus 4:22). And since they wanted to prevent them from serving Hashem, their firstborns died and they could no longer enslave them (Shemot Rabbah, Parsha 5, Siman 7).

B. They were killed at night, because they enslaved Israel even at night (ibid. Parsha 18, Siman 9).

Here are some of the reasons and explanations for the plagues, which are part of the story of the Exodus from Egypt. In this manner, one should add and expand in explaining the other parts of this mitzvah, in a clear and pleasant language, and whoever elaborates on it is praiseworthy.

And just as in the days of our exodus from Egypt, He will show us wonders, and we will merit to eat from the sacrifices and the Pesachim with joy and gladness, in our Holy and splendid Temple, speedily in our days, Amen.