The Short Second Life of Bree Tanner by Stephenie Meyer
I watched his body change. He crouched on the roof, on hand gripped the edge. All that strange friendliness disappeared, and he was a hunter. That was something I recognized, something I was comfortable with because I understood it.
I turned off my brain. It was time to hunt. I took a deep breath, drawing in the scent of the blood inside the humans below, They weren't the only humans around, but they were the closest. Who you were going to hunt was the kind of decision you had to make before you scented your prey.
It was too late now to choose anything.