Because they would throw the Israelites into dungeons, God brought darkness upon them, the darkness of hell, so that they had to grope their way. He that sat could not rise up on his feet, and he that stood could not sit down. The infliction of darkness served another purpose. Among the Israelites there were many wicked men, who refused to leave Egypt, and God determined to put them out of the way.
But that the Egyptians might not say they had succumbed to the plague like themselves, God slew them under cover of the darkness, and in the darkness they were buried by their fellow-Israelites, and the Egyptians knew nothing of what had happened. But the number of these wicked men had been very great, and the children of Israel spared to leave Egypt were but a small fraction of the original Israelitish population.