Evil is before you - i. e. “your intentions are evil.” Great as the possible infliction might be, Pharaoh held it to be a less evil than the loss of so large a population.
Related Commentaries of Exodus 10:10
Exodus 10:10
10 And he said unto them, Let the LORD be so with you, as I will let you go, and your little ones: look to it; for evil is before you.