He doesn’t hurt him at first. Doesn’t even want to hurt him, really. Sam Winchester is an errant child, a hapless puppet caught in the middle of a scenario he can’t possibly comprehend. So he maintains Nick’s familiar form (a troublesome task, twisting his true form into this tiny, plebeian shape), contemplates whether he’ll punish or reward that little red-haired witch when he gets out, and pries into the chinks in Sam’s armor, the ones he’s already so well acquainted with. He knows emotional torture will break him before physical torture, and he’s got the tools for that.
But his logic, flattery, persuasion, and threats have no impact. The fool stubbornly stands in the corner of their tiny cage, repeating the same insufferable no, and finally Lucifer’s control shatters. He explodes into his true form and howls his rage. There is an answering shriek of agony and terror, and when he folds himself into Nick’s form again, he sees Sam crumpled on the floor, eyes and ears bleeding, every bone in his body snapped in two.
“Oh, Sam,” he says. “I’m sorry. I lost control there for a minute. I didn’t mean for that to happen.” He kneels next to the broken man. “Don’t worry, I can fix this.” He starts with the tiny bones in his foot, slowly and painstakingly repairing each one. Slowly, slowly. “Of course, it would be faster from the inside. It’s going to take hours this way. Maybe days.” As he listens to Sam Winchester whimper in fear and pain, he thinks on the little witch again and decides, yes, it will be a reward.
(FYI, the title is from O Holy Night, one of my favorite Christmas carols.)