Derek was torn. He was hurting, and wanted to be alone to lick his wounds. He wanted to comfort his hurting slave. And he was also angry at Stiles for demanding that he act 'like a Master', as if what the werewolf was doing wasn't exactly that. He was a Master dealing with slaves who had disappointed him, even if they hadn't outright disobeyed him.
"Don't lecture me on how a Master should behave when you haven't the slightest idea how to act like a slave," he snapped. "I know exactly who's to blame for this, and it's not Isaac." It was himself. But that didn't mean he couldn't be angry, and hurt, and disappointed.
He glanced at Isaac again, wanting to comfort the slave but not at Stiles' behest, and then turned his back, heading down the hall to his bedroom.