As Moses gets the final instructions about the last of the plagues, this one hits the people of Egypt in a powerful way.  It is the killing of the firstborn.  As Egypt kept the children of Israel (God's firstborn) as slaves, aways from the life of worshiping God, so would God kill the firstborn of Egypt.      The same holds true today.  As God's people, the church are inundated with foreign gods to be worshiped, imposed upon by the power of the State, God will not tolerate the enslavement of His firstborn, the church.