"Good going, puppy." Cooper means that in equal parts sarcasm and sincerity. As the law student has probably figured out, Cooper hardly ever says anything without adding in a witty word or two. It's just the way he's made.

"If you don't got that much blood on your hands, you're already doin' better than I was at your age." At age twenty-five, Cooper had already been an experienced killer, remorseless and full of blood lust. He hadn't really snapped out of it until the late 1800s.