Derek let the kiss deepen, get hot and filthy, tongues warring, a foreshadowing of the ways in which Stiles was going to get filled. It was when they were like this that the werewolf wondered how Stiles could ever compare himself to Isaac. Isaac was just as greedy, just as needy when it came to wanting Derek's mouth and hands and cock, but he was never as demanding, never challenged--accidentally or on purpose.
It had been a long time since Derek had been challenged by a slave, and his last memory of it wasn't pleasant. But there was some part of him, a part that he denied existed, that wanted this. It wasn't just a matter of wanting Stiles to be Stiles, to want him free to be himself in a way that Isaac suppressed. There was a part of Derek, a self-loathing, guilty part, that thought he deserved this. That needed the confirmation that he was a horrible Master.
These thoughts were in the background, a secondary voice that sounded suspiciously like Kate, as he fingered the boy open, adding a second finger, then a third.