Glasse isn't corrupting light at all. What makes light good or darkness bad? Nothing at all. You only assume that it's that way because nearly every story in popular culture makes it that way (Harry Potter, etc.). Glasse harnesses light as a weapon because that's what he uses, there's no corruption involved, light is by its very definition as hurtful as it is helpful. (Sunburn, anyone?) Darkness is similar in nature, it can be used to hide, protect, guard, whatever. It's the same principle as a bad guy using fire or a good guy using ice, just replace fire and ice with different elemental properties. Voillah.
There's a more story-driven explanation to why certain characters use X power, but that isn't valuable for this discussion. All that matters is that X character uses Y power, and that he/she uses it is entirely independent of 'alignment' if you will.
"I'll come to Florida one day and make you look like a damn princess." ~Hep