Violence Quotes in Dark Places

How we cite our quotes: (Chapter.Paragraph)

Quote #10

For the first time, Calvin Diehl thought of himself as a murderer. He fell back in his seat and bellowed. (39.8)

Why does Calvin Diehl, a man who has killed many people, only consider himself a murderer when he thinks about killing Debby? And why does it bother him so much? It's like he's a vegetarian who accidentally ate a cheeseburger.