After months and months of rebellion, Pharaoh finally admitted he was wrong. The Dos Zakainim explains that the plague of barad moved Pharaoh more than any other. And it was because of one factor: Moshe had warned him that the hail would kill anything living. Again and again, Moshe cautioned Pharaoh to take his livestock and his slaves inside. Because Pharaoh was repeatedly warned to save the living creatures, he was moved and recognized his error.