The Ben Ish Chai writes about the concept that light comes from darkness. Quoting the verse (Tehillim 94:12) that “happy is the one afflicted by H-Shem,” he explains that affliction brings about teshuva. Similarly, all eyes have a light part and a dark part. The light in our vision actually comes from the dark part of the eye. This is what the king meant by saying “like is good in your eyes.” This evil that was in Haman’s eyes will bring out good in the end.
Megillas Sesarim focuses on the word “licha” (“your”), whose gematria (30+20=50) is equal to the size of the gallows Haman prepared, fifty cubits (Esther 7:10).