I agree with Buzzard. A kill should always be a deliberate action, either on someone who has already been taken out, or by inflicting is as the taken out result, if the player so chooses.
The better question is, what would an enforced kill at this point accomplish? Do you want to stop them from interrogating the thug? There are lots of non-lethal ways to do so. Throw him in a coma long enough to be of no value. Let them interrogate him and he doesn't know anything useful. Let him loose his memories from the blow and see how the players deal with that. A kill is not really necessary, if it doesn't help the story along, especially, if there are much more interesting ways to deal with the situation. Maybe he really has important information, and the wizard is now pondering, if he should look for the answers while the guy is in a coma.
And to enforce things like that, it is always within your rights as a GM to compel an appropriate aspect. But really, only do so, if it would be entertaining, just saying "Your superstrong, so the guy is dead" is pretty boring. Or maybe kill the guy, but let him haunt the PC now, so maybe they can still access the information, but they would have to help the ghost somehow, creating a new storyline. But again, the player(s) should be cool with this, it doesn't help anyone if you just shove things in their face to piss them off.