Was he enjoying this? Not the pain. He’d pass on the pain. In fact he’d passed out on the pain. But there was that small part of him he’d heard sometimes during strenuous arrests after long chases, the part that wanted to punch and punch long after punching had already achieved its effect. There was a joy to it. He called it the beast. It stayed hidden until you needed it and then, when you needed it, out it came. Pain brought it out, and fear. He’d killed werewolves with his bare hands, mad with anger and terror and tasting, deep inside, the blood of the beast… and it was sniffing the air.
— Terry Pratchett, Night Watch