God does not leave us to fight our battles alone. He is with us and he gives us the power to overcome sin and darkness in our lives. While the Egyptians were left in total darkness, the land of Goshen where the Hebrews lived was filled with light. The withdrawal of light speaks of punishment for sin. If we drive God out of our lives we are left in spiritual darkness.
Sin leads to darkness and eventual death.
In the last plague and final confrontation between Moses and Pharoah we see the ultimate judgment and victory of God. God passes through Egypt as a destroyer, killing the firstborn of man and beast, but sparing those who are sealed by the blood of the lamb. The firstborn, like the first fruits belonged to God. This was a representative action of God. In striking the firstborn of Egypt, God signified that all were guilty.