"What makes a spell dark?"

"W-what?!" Her professor yelped.

"What makes something dark? Is it intent? If I use lumos to blind you am I casting a 'dark' spell?"

"I-I d-don't," he stammered at a loss.

"What about the Unforgivables? If someone asks you to kill them, is it really unforgivable? If they're in pain? Wouldn't you be helping them?"

"W-we're t-t-talking about v-v-vampires," he muttered.

"Alright," she continued, "what makes a creature dark?"

Quirrell whimpered as 20 pairs of eyes focused on him.

The girl just smiled.